3.5 Broad area effect
In the previous section, the basic sub-threshold figure of merits are investigated and the obtained results are reported and commented.
Hereafter the focus will be posed on the effect brought by the large width of the active region. By design, every device in the set has a 50 µm width and, as a result, a multi-modal emission behaviour is reported.
Due to the broad area of the active region, several high order modes are guided, in these cases an issues that could arise is the collimation of the beam-profile within the instrumentation used for measuring, problem which is strictly related to the spatial coherence of the emitted field.
The investigation are performed in parallel with another identical devices, same structure and material, same doping concentration, but with an active region width of 5 µm.
3.5.1 Near-field beam profile
In order to verify the actual multi-modal nature, a near field measurements is carried out. A first measurement was performed by means of a setup in which the laser is brought above threshold and focused inside a NIR CCD camera, this procedure gives as a result the image of the near-field emission beam profile.
(a) Group C, 50 µm broad area. (b) Group C, 5 µm narrow ridge.
Figure 3.11: CDD camera maps for the near beam shape.
In figure 3.11 the measurements from the CCD camera are reported. The same setup is used for both the devices, the distance between the facet and the CCD camera objective kept fixed at ∼ 10 cm and the measurements were performed at equal condition of temperature and room illumination. Therefore, within a certain error, it can be asserted that the plots 3.11 (a) and (b) are on scale.
Post-process analysis
The results clearly show how the beam shape of the wide ridge devices is spread along the longitudinal axis, while on the vertical one is narrow and similar to a single mode behaviour. On the other hand, narrow ridge device show a well de-fined single mode beam profile, which has a Gaussian-like shape in both cardinal directions.
In order to measure and verify the beam profile with a second technique, a single-mode fiber has been stepped through the laser beam using a step-by-step electro-mechanical engine. Thereby, for every step the light coupled into the fiber is mea-sured by a power meter. By properly choosing the step and the acquisition time, the exact profile shape is measured
(a) Group C, 50 µm broad area. (b) Group C, 5 µm narrow ridge.
Figure 3.12: Beam profile fiber scanned along the x-axis.
Figure 3.12 shows the results of the process described above. The scans were performed for both laser in both direction, in here is reported a couple of scan performed where the broad area effect takes place. As we can see, for the 50 µm width device depicted in3.12(a), high order lateral modes give rise to several power peaks in the emission field, while on the other axis the behaviour of the field is ac-tually the same of the narrow ridge device, shown in 3.12 (b). This last case, is reported to have Gaussian-like beam profile in every scan directions and this yield a perfect single mode emission. Moreover, it is noticeable how, for increasing value of injection current, the high order mode arise and start to be more relevant in the profile shape.
Data shown in figure3.11and3.12are raw, no fitting and post process are involved.
The characterization of the multimodal nature of our devices is a crucial step in the whole investigation, it allowed to take into account that, the measurements per-formed on the interferogram and on the amplified emission spectra below threshold, were not performed on the central mode, but also some of the higher order modes
3.5 – Broad area effect
were involved.
In order to quantify possible effect on the gain and related figures of merit, some measurements already performed and discussed in section 3.2 were ran again.
3.5.2 Far-field filtering
In order to screen the high order lateral modes, a filtering system was build up. The filtering of higher order modes below-threshold is not trivial at all: the set-up needs to be aligned and optimized for the maximum power detection, ensuring that only the central peak is measured. This can be a challenging task, when considering a beam profile as depicted in figure 3.12 (a).
In order to filter out everything but the central mode, two experimental solutions were approached. The first one involved the coupling of the output beam in a single-mode fiber which would act as a filter itself. A second approach involved a classical pin-hole far filtering setup. The scheme and concept of the latter, can be found in [31], where the same issues are investigated for an InGaAs-GaAs quantum well heterostructure.
(a) Details of the ASE spectra. (b) Effect on gain for two different devices.
Figure 3.13: Far-field filtering measurement results.
The comparison were made by running the acquisition of the ASE with and without filtering, for the same device on the same setup with the same working condition. Due to the amount of technological complication involved, the measure-ment is performed on the 1.5 mm device from group C device at injection current density of 2.40 A/mm2, just to be able to show the direct physical effect carried by the higher order modes. The spectra is chosen to be the one just below threshold, in order to maximize the power output and simplify the alignment procedure.
In figure3.13 (a) exemplary zooms into the ASE spectra for filtered and unfiltered
Post-process analysis
measurements showing 7 modes are depicted, due to the filtering, the fringe pro-file of the ASE is modified, the minimum in the Fabry-Perot resonance peaks is reduced. Bearing in mind the discussion about the retrieval techniques in section 3.2, in particular equations3.6and3.8, we recall the importance of the value of the valleys in the evaluation of the gain spectra. The high order modes cause a filling of the bottom ASE spectra, which brings an higher than expected Imin value, this results finally in a down-shifted gain spectra as depicted in figure 3.13 (b).
Also in this case, the measurement were performed on the very same devices, with the same carrier injection but different active region length, in order to verify if the mismatch between gain spectra, discussed in section3.3.2, can be influenced by the broad area effect. In figure 3.13 (b) the comparison is shown: the shift seems to be rigid, the effect of higher order modes is stronger on the tails of the Lorentzian shape, filtering led a less broad peak but has no effect on the central wavelength of emission, which is unvaried. But the issue remains, the filling of the ASE bottom does not contribute to the gain spectra mismatch.
Appendix A
Backup images
Backup images