Introduction
Modern high bit rate optical communication channels now use polarization, amplitude and phase of the optical carrier to encode digital information. The future of optical communication requires Optical Arbitrary Waveform Generation (O-AWG). The ability to produce any kind of optical waveform, through the use of an optical modulator and an electrical AWG requires better accuracy and reliability than ever before. O-AWG’s require full control of an optical modulator, including the ability to bias at any arbitrary point on the modulator's electrical-to-optical transfer functions. The needs of the research and development community are ever changing and a truly arbitrary O-AWG will be as necessary to the optical R&D engineer as is the electrical AWG is for the RF engineer.
What is O-AWG and why is it necessary?
Optical Arbitrary Waveform Generation is an instrument that enables the user to fully configure and control the optical electrical field. This would include controlling its carrier frequency (wavelength), amplitude, phase and polarization over time. The user can define their arbitrary optical waveform without any thought or concern to how the optical waveform is generated.
As of today, engineers spend a huge amount of time building what is effectively their own O-AWG; characterising each aspect of their home made O-AWG, building models of the components and correcting their non-ideal properties. All this before the first optical waveform can be synthesised. If the engineer’s goal is to test new modulation formats or explore different pulse shaping techniques in a given system architecture, or test new algorithms to generate channel pre-distortion or even self-phase and cross phase mitigation techniques, then all this time is a distractor from their main goals.
In its most flexible and general form, the O-AWG requires four building blocks as shown in Figure 1: a laser to generate the optical carrier, an electrical AWG to generate 4 electrical waveforms over time, an optical modulator to impart the electrical waveforms onto the optical carrier and finally a very reliable and accurate bias controller to control the optical modulator.
Figure 1 - An example set up of Optical Arbitrary Waveform Generation
Why is ABC necessary?
The most flexible optical modulators for general O-AWG purposes are optical modulators that are a class of modulators commonly referred to as dual-polarization IQ-modulators (DP-IQMOD). These modulators consist of 6 Mach-Zehnders structures in a single package (see Figure 2). Each of the Mach-Zehnders must be accurately controlled and biased at its optimal points to ensure reliable data conversion from the electrical domain to the optical domain.
Figure 2 - Schematic diagram of a DP-IQ Modulator
Auto-bias control of optical modulators has always been a critical part of optical communications ever since the first crystal (Lithium Niobate) Mach-Zehnder modulators were invented and this remains true today with the semiconductor (Indium Phosphide) Mach-Zehnder modulators. Since Mach-Zehnder modulators rely on the electro-optic effect to modify the phase in each arm of the interferometer, they are fundamentally sensitive to the wavelength of the optical carrier as well as being sensitive to thermal effects from both the slowly varying environmental conditions and localized RF heating from the electrical inputs.
Until recently, high quality auto-bias control of dual-polarization IQ-Modulators has been only available to the large network equipment manufacturers through proprietary design. The lack of availability of flexible and reliable ABC for research and academic purposes has been a hurdle to the development of next generation modulation formats. Of course auto-bias control for IQ-modulators have been available for the research community for a few years, however, these auto-bias controllers only behaved well with simple electrical drive signals and well defined electrical drive levels that avoided the control null points. Packets of data with different modulations formats could not be transmitted without the ABC losing lock.
Where to now?
The first step into achieving a commercially available O-AWG has been achieved by Coherent Solutions Ltd with the introduction of the IQABC, an automatic bias controller for IQ-modulators (see Figure 3). Researchers now have a robust and reliable automatic bias controller that is insensitive to electrical drive signals. Regardless of whether the electrical waveforms are NRZ, PAM4, OFDM or packets with varying data formats, the IQABC maintains its bias points. The IQABC can be controlled from the IQSignalManager software or it can be fully controlled with SCPI remote control commands.
Figure 3 - IQABC from Coherent Solutions
The IQABC is also available as part of the IQTransmitter-40G which encompasses the IQABC along with a laser, a 40GHz Dual Polarization IQ-modulator and RF linear amplifiers (see Figure 4). The IQTransmitter simplifies yet again the equipment necessary to generate arbitrary optical waveforms, and with its industry leading high bandwidth, it ensures the generated waveforms are of the highest quality even at data rates exceeding 56GBaud.
Figure 4 - IQTransmitter from Coherent Solutions with built-in automatic bias controller
With the availability of versatile and robust automatic bias controller, constructing an O-AWG has just become easier. Easier generation and maintenance of coherent modulation formats will help the industry explorer wider options such as the 32QAM format shown in Figure 4, leading to faster and more robust data transmission. Where there is demand, supply is sure to follow and we may soon be seeing O-AWG as an integrated product from leading test instrumentation vendors.
Figure 5 - Optical 32QAM signal generated using IQABC
About Coherent Solutions
Coherent Solutions combine extensive knowledge and experience with agile R&D teams to bring you highly advanced OEM and ODM solutions. They specialize in coherent optical communication test and measurement instruments from signal generation to characterization. Visit their website to see the leading edge range of products and equipment development services.
