Tunable DWDM Lasers – a short overview

by www.fiber-mart.com

A tunable laser is a laser whose wavelength of operation can be altered in a controlled manner. While all laser gain media allow small shifts in output wavelength, only a few types of lasers allow continuous tuning over a significant wavelength range.

 

In order to enable high-capacity optical networks, DWDM systems which use a single optical fiber for optical signals of several different wavelengths are being utilized.Wavelength tunable optical transceivers are becoming important as components that enable ROADM –  Reconfigurable Optical Add/Drop Multiplexer – functionality in next-generation networks. These transceivers have the characteristic that their wavelengths can be switched between different DWDM channels while in use in the network. Tunable transceivers are only available in DWDM form because the CWDM grid is too wide. Typically, these tunable optics are for the C-Band 50GHz. Around 88 different channels can be set with intervals of 0.4nm, which is the 50G Hz band. These optics usually start from channel 16 up to 61 but this depends on the manufacturer of the Router/Switch and which channels it supports.

 

Working principle

Multiple individual lasers, are built into one piece of silicon.

 

Tunable Distributed Bragg Reflector (DBR) Laser

One of the earliest types of tunable lasers is the distributed Bragg reflector laser. More modern tunable devices still share the same basic concepts and can be considered an evolution of DBR lasers. Like in DFB laser a DBR introduces a periodic variation of the refractive index effectively generating a Bragg grating or reflector. The cleaved front facet of the device acts as a second mirror. Only the wavelengths in a specific relation with the Bragg period survive in the cavity. Tuning is achieved by injecting current into the Bragg reflector. This results in a modification of the refractive index, which causes the Bragg peak to tune to different wavelengths. The Phase Section is primarily designed for fine-tuning the output Wavelength. The tuning range of these devices is proportional to the maximum change in the refractive index, typically below 20 nm.

 

Grating-Assisted Co-directional Coupler (GACC) Laser

The grating assisted codirectional coupler (GACC) laser is very similar to a DBR in operation. The purpose of this structure is to extend the tuning range of a DBR. The tuning element is a pair of vertically stacked waveguides with different material properties and a grating. This change leads to a larger tuning range in excess of 60 nm.

 

Sampled Grating DBR (SG-DBR)

The sampled grating DBR is another variant of DBR laser whose main difference is the presence of a pair of grating mirrors at either end of the cavity. The gratings are periodically sampled or blanked out, which results in a sequence of equally spaced short grating bursts. Just as in DBR, the gratings can be tuned by current injection. It can be proven that by differentially tuning the mirrors it is possible to achieve a wider tuning range than with a simple DBR.

 

External Cavity Tunable Laser (ECL)

The main characteristic of this architecture resides in moving out of the gain cavity the wavelength selection device, which is typically a MEMS or a thermally tunable filter. There is no integrated grating in the laser cavity like in a DFB or DBR. Tunable Lasers fabricated with this technique are usually very high-power (13 dBm of output power) and have a high spectral purity (SMSR > 50 dB). Among the disadvantages, an ECL is usually very slow to switch from one wavelength to another (in the order of seconds), furthermore in MEMS-controlled ECL mechanical reliability is a concern.

 

The operating frequency may be defined by a frequency selective feedback element that is thermo-optically tuned by the application of heat from an actuator without substantially tuning the cavity modes. Configuration is controlled by the operating system software in use for the DWDM system.

 

Thermal compensation of laser resonators is a requirement in components that must operate robustly within the narrow absolute frequency bands of the DWDM specifications.

 

Application of tunable DWDM lasers:

 

Sparing

Use tunables to reduce the number of line cards needed to back up all the different wavelengths in a system.

Dynamic provisioning

The wavelength of the tunable transmitter can be changed once the system has been deployed.

Reconfigurable optical add/drop multiplexers (ROADMs)

A simple, more flexible architecture for ROADMs has been proposed, which relies on the use of both tunable lasers and tunable filters.

Optical crossconnects

Tunable lasers can remove wavelength-blocking issues in OXCs.

 

Dynamic restoration

When a DWDM channel fails, a tunable laser could automatically restore service