Tunable Lasers

Abstract

Tunable lasers as the name suggests are lasers whose wavelengths can be tuned or varied. They play an important part in optical communication networks. Recent improvements in tunable laser technologies are enabling highly flexible and effective utilization of the massive increases in optical network capacity brought by large-scale application of dense wavelength division multiplexing. 

The outlined approach of full-band tunable lasers based on DFB technology meet the existing performance specifications and possess a cost structure that will ultimately displace static- wavelength DFBs, making tunability ubiquitous.

Introduction

Tunable lasers as the name suggests are lasers whose wavelengths can be tuned or varied. They play an important part in optical communication networks. Recent improvements in tunable laser technologies are enabling highly flexible and effective utilization of the massive increases in optical network capacity brought by large-scale application of dense wavelength division multiplexing.

In a wavelength-division multiplexed (WDM) network carrying 128 wavelengths of information, we have 128 different lasers giving out these wavelengths of light. Each laser is designed differently in order to give the exact wavelength needed. Even though the lasers are expensive, in case of a breakdown, we should be able to replace it at a moment's notice so that we don't lose any of the capacity that we have invested so much money in.

Tunable Lasers Enable Next-g Capabilities

Beyond the ability to address today’s problems, tunable lasers will play a central role in allowing service providers to offer entirely new value-added services and generate new revenue streams.  Next-generation services and functionality will initially be implemented in a new generation of optical add-drop modules (OADMs) and optical cross-connects (OXCs) that incorporate tunable lasers.

Current-generation OADMs, which rely on fixed-wavelength lasers, are limited in their ability to add channels to the network. Changing traffic patterns, customer requirements, and new revenue opportunities require greater flexibility than static OADMs can provide, complicating network operations and planning. Incorporating tunable lasers removes this constraint altogether by allowing any channel to be added by the OADM at any time. With the deployment of tunable line cards at OADM sites, sparing and restoration capabilities become more economical as well. Optical cross-connects (OXCs) represent another opportunity for tunable lasers to improve network system efficiency. Line cards with widely tunable lasers covering the full C band enable physical-layer provisioning and switching in the optical domain, allowing an “any channel to any channel” connection to be made at network nodes. Widely tunable lasers simplify OXC planning, since all wavelength channels are available from any tunable line card. Even if a particular channel is already in use, wide tenability allows a18   flexible, potentially automated determination of wavelength conversion, which further simplifies  system planning by complementing and adding value to electrical-level provisioning, grooming, and switching. While today’s networks are large and complex, future networks are expected to be greatly simplified and more purely optical in nature, with significant reductions in both capital and operating costs. One scenario shows a “mesh” architecture in which nodal points on the network are designed to route signals on the basis of wavelength. In this example, tunable lasers can be deployed to route signals to their destination on the basis of wavelength. Tunable lasers will play a key role in these advanced telecom networks, since this type of architecture is unlikely to be fully realized with current-generation fixed-wavelength or even narrowly-tunable lasers.

The Distributed Feedback Laser

Among the most common diode lasers used in telecommunications today are distributed feedback (DFB) lasers. They are unique in that they incorporate a diffraction grating directly into the laser chip itself, usually along the length of the active layer (the gain medium).As used in DFB lasers, the rating reflects a single wavelength back into the cavity, forcing a single resonant mode within the laser, and producing a stable, very narrow-bandwidth output.

DFB lasers are tuned by controlling the temperature of the laser diode cavity. Because a large temperature difference is required to tune across only a few nanometers, the tuning range of a single DFB laser cavity is limited to a small range of wavelengths, typically under 5 nm. DFB lasers with wide tuning ranges therefore incorporate multiple laser cavities.

Better Use Of Expensive Network

Bandwidth Tunable lasers provide an advantage over fixed sources even when  service providers employ an alternative “hot-backup” approach to sparing  that  is, maintaining idle channels which are only activated when a backup is required.  In this application, upto 50% of the system bandwidth can be rendered unusable when using fixed wavelength lasers because network carriers must maintain a spare channel for each wavelength used. With tunable lasers, however, only a small  number of line cards are held in reserve slots, since each spare can tune to any  required wavelength. Thus tunable lasers can restore usable system bandwidth to 90% or better.

Moreover, in the event of a channel failure, a tunable backup card can be quickly configured to resume communications, providing nearly seamless restoration in the event of malfunction and allowing SONET protection to be implemented entirely in the optical domain. As a further guarantee of service continuity, tunable lasers therefore present additional opportunities for revenue generation.

Conclusion

Recent advances in tunable laser technology have brought the promise of tunable networks into clear focus. Widespread adoption of tunable lasers will not only eliminate logistical and inventory problems and the associated costs that result from fixed-wavelength line cards but will also enable novel network architectures with dynamic functionality such as dynamic add-drop, thus enabling new value-added services and creating new sources of top-line revenue for system providers.