Tuneable lasers: The elusive trifecta

Integrated tuneable laser assemblies serving in the C- and O- bands excel on three fronts by offering the unique combination of wide tuning, fast-nanosecond switching and a narrow linewidth.

BY CAOLÁN MURPHY, SHANE DUGGAN, GAURAV JAIN, CRISTIAN VARGAS, AND DESI GUTIERREZ FROM PILOT PHOTONICS

TUNEABLE LASERS are now established as a key enabling technology in a wide range of applications, spanning various fields from optical communications to sensing.

In telecommunication infrastructure, particularly in the long-haul and metro networks, widely tuneable lasers with a low linewidth are indispensable components. When incorporated in coherent transceivers, their low linewidth enables increased data rates by supporting advanced modulation formats within dense wavelength-division multiplexing systems. The wide wavelength tuneability is a valued asset, simplifying network design and management by enabling resources to be dynamically assigned and optimised while reducing inventory requirements.

Meanwhile, in optical fibre sensing applications, such as fibre Bragg grating sensing, fast-swept tuneable lasers are used to interrogate sensors with Bragg gratings inscribed into optical fibres. These gratings reflect specific wavelengths of light, based on the strain and the temperature applied to the fibre, to create a distinctive spectral signature. When lasers offer fast switching and are widely tuneable, this enables precise and fast wavelength sweeps across the fibre Bragg grating sensor’s bandwidth to ensure real-time, accurate readings of strain or temperature.

Figure 1. Tuneable laser form factor offerings: Photonic Integrated Circuit (PIC), the laser cavity consists of a gain section providing optical gain in either the C or O bands, two ring sections that act as tuneable mode selectors through the Vernier effect, and a common phase section to allow finer tuning of the laser cavity mode; 14-pin butterfly package; OEM benchtop unit; nano-integrated tuneable laser assembly package.

Fast tuneable lasers have also played a key role in the development of many optical switching architectures, involving rapid adjustments to the laser wavelength to quickly switch or route optical traffic without having to convert signals to the electronic form.

There is no doubt that the versatility of the tuneable laser has enabled widespread applications. However, today’s commercially available widely tuneable lasers still fall short in some areas. In particular, they struggle to serve in applications requiring phase-sensitivity and very fast-switching/sweeping.

For instance, for frequency-modulated continuous-wave lidar, a cutting-edge technology in autonomous vehicles and remote sensing, they are not adept at providing the low linewidth and ultra-fast sweeping capabilities needed to precisely measure distances and velocities. Additionally, in systems that employ coherent optical time-domain reflectometry and optical frequency-domain reflectometry, rapid, phase-sensitive tuning is critical for accurate detection of disturbances or variations along the optical fibre, or for other devices under test. If optical packet or optical burst systems are to re-emerge, they will have to align with the demands of today’s high-speed data transmission, which requires not only very fast wavelength switching, but the ability to handle phase-sensitive modulation formats.

The three desirable characteristics for tuneable lasers, as well as the typical design rules required to achieve them. Few designs are able to achieve the combination of wide tunability, fast switching and narrow linewidth.


Unfortunately, today’s widely tuneable semiconductor lasers are not great allrounders. Typically, they offer either a narrow linewidth or fast tuning – but not both. An example is the sampled-grating distributed Bragg reflector laser, which is capable of tuning across tens of nanometres using the Vernier tuning effect, thanks to a current-injection-based tuning mechanism that enables swift optical switching. However, this comes at the expense of increased phase noise and broader linewidths, rendering this design unsuitable for the aforementioned applications.

The alternative is to make use of an equivalent thermally tuned laser. This class of laser is capable of realising similar tuning ranges, while boasting low linewidths. However, fast electronic tuning is sacrificed. Similarly, SiN ring-resonator-based devices major in ultra-low linewidths via thermal tuning, but dramatically reduce switching speeds, also limiting usage in these applications.