Widely Tunable Lasers
A tunable laser diode offers the possibility to change the laserwavelength by a seperate tuning current. Therefore at least two currents, one for laser operation and one for tuning operation, are needed for such devices (see figure 1). Devices with a tuning range exceeding 10 nm are called widely tunable.
Figure 1: Scheme of a tunable edge emitting laser.
Tunable laser diodes are essential components in many technical devices and applications just like
· WDM- or DWDM-Networking
· Fiber Bragg Gratings
· Tunable Diode Laser Sensing for Gas Tracing (TDLS)
hence due to the high variety of fields of application tunable laser diodes are becoming more and more important.
The basic aims by designing and producing a tunable laser are high output power, a wide tuning range and low production costs.
Generally there are two device concepts popular, tunable edge emitting and tunable Vertical Cavity Surface Emitting Laser (VCSEL). Both concepts have been investigated and high quality devices have been achieved at the WSI.
Widely Tunable Edge Emitter
The tuning principle of edge emitting lasers is based on changing the refraction index of one layer by injecting carriers, which is known as plasma effect. If this layer is part of a DFB grating, which works as a longitudinal mirror and is needed for single mode operation, the optical thickness of the grating period length changes and therefore the laser wavelength is shifted. It should be mentioned that the plasma effect decreases the refraction index with increasing carrier density; hence, the laser wavelength will be blue shifted. A simple device application, called Tunable Twin Guide Laser (TTG-Laser) is shown in figure 2. A continuous wavelength range of about 8 nm is reachable by increasing the tuning current It.
Figure 2: The basic structure of a TTG-Laser (left) and the corresponding continuous tuning scheme (right) are illustrated.
For a further extension of the tuning range more complex devices are needed. In figure 3 the tuning scheme of a so called Sample Grating TTG-Laser (SG TTTG-Laser) is displayed. The DFB-Gratings is divided into two Sample Grating sections with respectively tuning currents. Therefore are now two tuning operations available:
· Changing simultaneously both tuning currents results in a continuous wavelength range like in the case of a normal TTG-Laser.
· Wide quasi-continuous wavelength-tuning is accomplished by utilizing the Vernier-effect. The two SG's, which posses slightly different sampling periods (F and R), generate comb-like reflection spectra. Lasing takes place when reflections peaks from both SG's are lined up. Large wavelength jumps, so-called supermode hops, are obtained by tuning one reflector while leaving the other one unchanged.
Figure 3: A 2D drawing of SG-TTG Laser (left) and the tuning behaviour of this device (right) are shown.
As shown in figure 3, such device offers a quasicontinuous tuning of the laser wavelength by using only two control currents (It,1 and It,2). The accessible wavelength range is around 40 nm, with output power between 10 and 20 mW and SMSR > 35db.
Furthermore another tuning design has been investigated called Vertical Mach Zehnder Laser (VMZ-Laser) . The tuning, as the name already implicates, is based on a vertically integrated Mach Zehnder Interferometer section, which offers the advantage to pick certain wavelengths with a defined spectral separation. This is very useful for WDM or DWDM application where only defined wavelength channel are accessible.
Figure 4:Longitudinal section (a) and cross-view SEM picture (c) of a VMZ-Laser structure. The discontinuous tuning characteristic is showed in figure (b).
The wavelength selection of the filter (see figure 4) is accomplished by interference at the transition between interferometer section (two supported modes) and single-waveguide sections (one supported mode). At certain wavelengths, constructive interference leads to high filter transmission allowing laser emission to take place. By injection of carriers into the tuning layer, the transmission maxima can be changed (plasma effect), and thereby the laser wavelength can be shifted. A tuning range of 32 nm, with an output power of 12 mW and 56 wavelength channel could be realized.
Besides the compactness, the monolithic integration and simplicity of tuning mechanism, the high production cost of these devices is a serious drawback of edge emitting lasers. Therefore our group is now focused to realize widely tunable VCSEL, which offer lower production costs.
Widely Tunable Surface Emitting Laser: Introduction
The basic concept of widely tunable Vertical Surface Emitting Lasers (VCSELs) is the implementation of a moveable mirror to the basic laser structure, shown in figure 5.
Figure 5: The basic concept of a tunable VCSEL is illustrated. The top mirror of the VCSEL is removed and replaced by a moveable membrane (shown here) or cantilever.
By moving the mirror, the laser cavity length can be changed and therefore the laserwavelength. With such a technique tuning ranges from 30 to 60nm have been achieved, whereas standard VCSELs can only be tuned in the range of 10nm by heating up the active region by current.
Device Concept: MEMS - VCSEL
In the past years several concepts have been developed for the realization of a widely tunable surface emitting laser, mostly with electrostatic or piezoelectric actuated membranes or cantilevers as moveable mirror.
Here at the Walter Schottky Institut in coorporation with TU-Darmstadt a novel electrothermal tuning concept has been developed, shown in figure 6.
Figure 6: The fusion oft he GaAs based mirror membrane and the half VCSEL is illustrated.
In this concept a GaAs based curved mirror-membrane, consisting of AlGaAs/GaAs l/4 layers with a certain compressive stress gradient (achieved by adding GaInAs layers), is fused to the InP based “half” VCSEL (the notation “half” is used, because of the missing top mirror) and actuated by heating it up (Joule heat), as shown in figure 7.
Figure 7: The tuning principle of the MEMS-VCSEL is shown.
Therefore the tuning principle is very simple and less effort is needed compared to electrostatic tuning. Another big advantage is the large tuning range of 30 to 60 nm. This device is called a Micro-Electro-Mechanical-System (MEMS) VCSEL.
The fused hybrid devices showed a high tuning range of up to 50 nm, with an optical fiber coupled output power between 0.5 and 2 mW at 1.55 µm. As figure shows, is the wavelength tuning proportional to the squared tuning current and therefore proportional to the Joule heating of the membrane as expected.
Finally, the high side mode suppression ratio (SMSR) of >40 db ensures single mode operation in the hole tuning range, which is needed for applications.
Unfortunately the fact, that this is a non monolithic device concept, evokes high fabrication costs compared to monolithic tunable DFB lasers.
Figure 8: The tuning range and SMSR of the MEMS-VCSEL devices are shown.
EU – Projekt “Subtune”
Based on the above presented results a EU project (7th Framework) called “Subtune” has started (Starting day 01.04.2008), in which the Walter Schottky Institut is responsible for the growth and processing of InP based “half” VCSEL. The other partners of the project are TU Darmstadt, Chalmers (Sweden), CEA LIST (France), Tyndall (Ireland), IRM (Switzerland) and CNR (Italy).
The project has the following aims:
· Covering a wavelength range from 0.85 to 2.10 µm
· Devices with tuning range up to 60 nm
· Development of high speed design for 10 Gbit/s
· Polarization control by implementing a Sub Wavelength Grating
· Monolithic Integration of the entire device