Lasers & Sources

Lasers for Communications: Optical injection locking brings back direct-modulation telecom lasers

March 5, 2015

A modulator-free, optical-injection-locking method brings the benefits of direct modulation back to the modern coherent telecommunications network.

Gail Overton

The principle of operation of the optical-injection-locked transmitter is shown (a) using constellation diagrams (in the complex plane) for 16-level QAM signal generation. Two amplitude-modulated signals are combined (Stage 1), making a signal with a constellation similar to that desired. By removing the carrier component of the signal via interference with the master signal (Stage 2), the desired modulation format signal is obtained. In (b), a sketch explains the difference between a free-running and an optical injection lock-assisted directly modulated semiconductor laser. Due to the large chirp associated with the directly modulated laser, the constellation consists of four rings rather than four points, making it unsuitable for IQ modulation. However, optical injection locking causes the four circles to collapse into two distinct points in the IQ plane.

While direct modulation of a semiconductor laser’s drive current enables fiber-optic communications at speeds of around 2.5 Gbit/s, higher-speed operation using direct modulation has historically been confounded by frequency chirp, forcing commercial 10 Gbit/s systems to use an external electro-optic modulator. For increased capacity (100 Gbit/s and beyond), coherent systems were recently commercialized that also use external electro-optic modulators to deliver complex modulation formats.

New research from the University of Southampton’s Optoelectronics Research Centre (ORC; England) and Eblana Photonics (Dublin, Ireland), however, has skirted this historical limitation through a modulator-free, optical-injection-locking method that brings the benefits of direct modulation back to the modern coherent telecommunications network.

Modulator limitations

Today’s high-speed in-phase-quadrature (IQ) modulation formats (such as quadrature amplitude modulation, or QAM) are modulated on an optical carrier using expensive, bulky, and difficult-to-integrate lithium niobate (LiNbO₃) IQ modulators. While indium phosphide (InP), silicon (Si), and gallium arsenide (GaAs) modulators are currently being researched, their performance is poor compared to LiNbO₃.

The Southampton modulation-free scheme uses optical injection locking (OIL) and direct modulation to enable high linearity, reduced power consumption, small footprint, easy integration, and cost-effective QAM transmitters. Direct modulation can also benefit any nontelecommunications applications that depend on controlling the phase properties of laser beams, such as coherent beam combination in fiber lasers.

IQ signal generation

First, three lasers are made coherent through OIL by locking two of them with the third one. Subsequently, the two optically injection-locked lasers are directly modulated with two independent data streams (representing I and Q). These two amplitude-modulated lasers are then combined (with their carriers 90° shifted with respect to each other) and interfered destructively with a master signal to remove the optical carrier part of the signal, thus generating QAM modulation. This OIL process not only makes the three lasers mutually coherent, but also suppresses modulation chirp. It can also significantly increase the modulation bandwidth to values up to 80 GHz.

In a proof-of-concept experiment, two optically injection-locked, directly modulated lasers (simple Fabry-Perot lasers) allowed—with help from temperature tuning—operation over almost the entire telecom C-band from 1530 to 1560 nm (see figure).

“Our work highlights the exquisite control that we have achieved over the optical field generated directly from a current-modulated semiconductor laser,” said Radan Slavik, principal research fellow at the ORC leading the research. The team aims to commercialize the technology.

REFERENCE
1. Z. Liu et al., Nat. Commun. 5, 5911 (Dec. 2014); http://dx.doi.org/10.1038/ncomms6911.

About the Author

Gail Overton | Senior Editor (2004-2020)

Gail has more than 30 years of engineering, marketing, product management, and editorial experience in the photonics and optical communications industry. Before joining the staff at Laser Focus World in 2004, she held many product management and product marketing roles in the fiber-optics industry, most notably at Hughes (El Segundo, CA), GTE Labs (Waltham, MA), Corning (Corning, NY), Photon Kinetics (Beaverton, OR), and Newport Corporation (Irvine, CA). During her marketing career, Gail published articles in WDM Solutions and Sensors magazine and traveled internationally to conduct product and sales training. Gail received her BS degree in physics, with an emphasis in optics, from San Diego State University in San Diego, CA in May 1986.