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Coherent optical signal generation with high performance arbitrary waveform generator

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By eeNews Europe

Over the years, capacity has been improved through the combination of multiple mechanisms:

· Installation of additional fiber optics cables

· Increase of the baud rate for a given link

· Improvement of the transmission characteristics of the fiber by reducing or mitigating the effects of attenuation and dispersion

· Multiplex of multiple signals in a single fiber by assigning different wavelengths to them

· Increase of the number of wavelengths transported by a single fiber by reducing the distance between them

· Addition of FEC (Forward Error Correction) techniques to enable faster connections in in lossy or dispersive environments.

The above improvements have been applied over time to optical signals using the traditional OOK (On-Off Keying) direct modulation scheme. Information is coded by controlling two states of the optical transmitter. Ideally, in one of them full power is transmitted while in the other zero power should be transmitted so only one bit can be coded by each symbol.

 

Figure 1. Spectral efficiency of optical transmissions may be improved by modulating both the amplitude and the phase of an optical carrier, which requires coherent modulation and detection. In this WDM link, four different wavelengths share the same fiber in a standard ITU 50GHz grid. Wavelength 4 is carrying a 10Gb/s signal using the traditional intensity modulation (or On-Off Keying, OOK). Part of the optical power goes directly to the carrier and does not transport any information. Carrier 3 is modulated using a QPSK modulation so 2 bits are transported by each symbol, doubling the capacity of the OOK-modulated channel in the same bandwidth. Capacity may be increased through the usage of more complex modulations or baseband filtering. Wavelengths 1 and 2 transport 28 Gbaud signals with 2 and 5 bits per symbol respectively.

 

As bits are transmitted faster and faster, optical signals drift away from the ideal conditions and issues like bandwidth (both optical and electronic) and dispersion (especially Chromatic

Dispersion) start to raise a wall impeding further improvements. Ultimately, the distance between contiguous wavelengths in a DWDM (Dense Wavelength Division Multiplexing) installation also limits the maximum baud rate an individual wavelength can be modulated (Figure 1).

Otherwise, beyond some limit, each wavelength would interfere with the adjacent wavelengths compromising the bit error rate level. Under these conditions, information is carried by a single optical parameter: power. Phase of the optical carrier is not typically important until its behaviour affects the capability to support transmissions at the required speeds. Line width, a form of phase-noise, or chirp (which changes in the wavelength during fast transitions) increase the bandwidth of each optical signal so the effects of wavelength-to-wavelength interference and dispersion grow. Presently, traditional OOK-based DWDM links carry up to 160 10Gbps channels (1.6 Tbps aggregated capacity) in a 25GHz ITU grid or up to forty 40Gbps channels in a 100GHz ITU grid. Commercial success of 40Gbps OOK modulated channels has been rather limited as it is only feasible at the expense of much higher cost and complexity due to the electronics involved and the need to apply powerful dispersion compensation techniques.

Spectral efficiency of optical transmissions may be improved by modulating both the amplitude and the phase of an optical carrier, which requires coherent modulation and detection. In this WDM link, four different wavelengths share the same fiber in a standard ITU 50GHz grid. Wavelength 4 is carrying a 10Gb/s signal using the traditional intensity modulation (or On-Off Keying, OOK). Part of the optical power goes directly to the carrier and does not transport any information. Carrier 3 is modulated using a QPSK modulation so 2 bits are transported by each symbol, doubling the capacity of the OOK-modulated channel in the same bandwidth. Capacity may be increased through the usage of more complex modulations or baseband filtering. Wavelengths 1 and 2 transport 28 Gbaud signals with 2 and 5 bits per symbol respectively.

Wireless and cable RF transmission systems faced similar problems in the past. Improvements in the capacity have only been possible through advances in modulation and coding techniques. Common trends have been the usage of higher order modulation schemes, where more than one bit is transmitted during a symbol period, and the exploitation of different kinds of orthogonality, where multiple independent messages can be sent simultaneously over the same link. The key for the success of both strategies is the ability to control the amplitude and phase of the RF carrier. The simplest way to send two independent transmissions over the same carrier frequency is to use orthogonal carriers with 90 degrees phase difference. In real-world implementations, both messages are typically synchronous. The two messages can be independently decoded at the receiver if the original orthogonal carriers are recovered and coherent detection is applied. Using combined amplitude and phase control, it is feasible to map an alphabet of M symbols (typically M=2N)

to M states of modulation or amplitude/phase combinations.

 

Figure 2. One way to modulate the amplitude and the phase of a carrier is a quadrature modulator. There, two baseband signals, called I (or In-phase ) or Q (Quadrature), modulate in amplitude two orthogonal carriers (90o relative phase) so any state of modulation can be accomplished, The same scheme may be implemented for optical carriers by using two Mach-Zehnder Modulators (MZM) in an arrangement known as “Super-MZM” cell.

 

A quadrature modulator (Figure 2a) is a typical implementation for an actual transmitter. There, phase and amplitude of the carrier are controlled by setting independently bipolar amplitude

levels over two carriers with 90° phase difference forming a Cartesian (thus orthogonal ) axis arrangement: the I (orIn-phase) and Q (or Quadrature) components. The location of symbols in the IQ plane is known as the Constellation Diagram. Popular modulation schemes such as QPSK (2 bits/symbol Quadrature Phase Shift Keying) or M-QAM (log2M bits/symbol Quadrature Amplitude Modulation) show symmetrical, square constellation diagrams although there are other configurations or constellation shapes. An important issue of coherent transmission systems is that good spectral purity is required as phase noise translates directly to symbol location inaccuracies and, as a consequence, errored bits.

On paper, the same approach to capacity improvements would be desirable to increase the spectral efficiency (the amount of bit/s that can be fitted in 1Hz bandwidth) of optical transmission systems. As this method requires controlling both the power and the phase of an optical carrier, such communication systems can be defined as coherent optical links.

Researchers and engineers require adequate tools to validate, diagnose, and produce their designs, prototypes, and products. The goal of Test & Measurement (T&M) manufacturers is to provide the appropriate tools. Stimuli devices, capable of generating optical and electrical signals with enough quality, repeatability, and accuracy, are necessary to test receivers and other components, systems and sub-systems, even entire networks. These signal generators must be able to generate perfect (“golden”) or impaired signals and they must be capable of emulating the effects of interconnections and transmission systems. The main considerations when evaluating the performance of an AWG are:

Sample Rate (SR): The maximum speed at what digital samples can be converted to analog samples by the DAC.

Analog Bandwidth (BW): The effective bandwidth for useful signals being generated by the generator.


Record Length (RL): Size of the waveform memory. It influences the longest non-repeating time window that can be generated at a given sampling rate.


Vertical Resolution (Res.): The number of bits that define a sample in the DAC. This specification influences dynamic range as quantization noise depends on it.


Number of Channels: The number of arbitrary waveforms that can be generated simultaneously (usually synchronously) with the same AWG device. Channel count can be increased in some instruments by synchronizing multiple units. Synchronization quality is a very important issue in

this kind of application.

 

Complex modulation requires multiple synchronous baseband signals (up to 4 in a Polarization Division Multiplex link). Synchronizing multiple AWGs is extremely important to obtain useful signals. Here, the recommended multi-instrument synchronization scheme for the Tektronix AWG70000 series is shown. With this arrangement, channel-to-channel skews better than 4ps can be obtained.

The recently introduced Tektronix AWG 70000 Series, with 50Gs/s , 20GHz RF Frequency, -80dbc SFDR and 16GB memory making it ideal for such applications.

About the author

Dean Miles is a Senior Technical Marketing Manager at Tektronix – www.tektronix.com responsible for Tektronix High Performance Product Portfolio. Dean has held various positions with Tektronix during his more than 20 years with the company, including Global Business Development Manager for Tektronix RF Technologies and Business Development Manager for Tektronix’ Optical Business Unit.


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