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Efficient multi-mode fiber to single-mode fiber coupling : High speed VCSEL characterization

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This article aims at illustrating the MMF to SMF coupling using a multimode scrambler, as introduced in a previous article, to couple a Vertical Cavity Surface Emitting Diode Laser (VCSEL) laser into a ZOOM Spectra high resolution Laser Spectrum Analyzer.

Indeed, VCSEL laser are really compact semiconductors laser diodes with large Numerical Aperture emission beam perpendicular to their top surface, as illustrated figure 1. VCSEL are generally fabricated by batch of millions units on a wafer using standard semiconductor technologies. Hence, as it is done for IC testing, the optical characteristics of each single VCSELS needs to be tested to make sure they meet their specifications.

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Figure 1 : Typical VCSEL layout with surface emission perpendicular to the surface (source : http://bit.ly/2lLWnA2)

 

As one can guess, this testing step can be very time consuming, especially since every single VCSEL has to be individually tested with an optical characterization instrument.

For these reason a high measurement speed and high sensitivity optical characterization tool, such as the SWIFTS-based ZOOM Spectra presented in figure 2 (30kHz measurement speed, 10nW sensitivity), would be a perfect tool to carry out the VCSEL optical characterization at the end of fabrication process.

 

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Figure 2 : ZOOM Spectra high resolution, high measurement rate Laser Spectrum Analyzer by Resolution Spectra Systems

The main issue with such setup lies in the fact that it is really difficult to couple a large N.A beam into the Single-Mode fiber input of the ZOOM Spectra instrument. For this reason, the use of a multimode scrambling system such as the one presented in the two previous articles seems a convenient way to allow the VCSEL characterization with ZOOM Spectra.  To illustrate this, a comparison between measurement performed with and without scrambling system has been carried out :

Direct VCSEL to ZOOM Spectra coupling :

In a first experiment a VCSEL laser has been coupled into a multimode fiber which was directly plugged into the ZOOM Spectra input. The typical output spectrum as measured by the ZOOM Spectra is the following:

Video 1: Video of a VCSEL Spectrum as measurement by a ZOOM Spectra with a MMF to SMF direct coupling

VCSEL to ZOOM Spectra coupling through a scrambling system:
In a second set of experiment, a spatial scrambling system – the Multimode Scrambler – SpeckleFreeTM (MMS-201)  by General Photonics ( see figure 3 ) – has been added to the coupling setup in order to more efficiently perform the MMF to SMF coupling so as to prevent any spectral loses.

Figure 3: Multimode Scrambler – SpeckleFreeTM (MMS-201) by General Photonics corporation

The video 2 shows the typical output spectrum as measured by a ZOOM Spectra when using a multimode scrambling setup

Video 2: Video of a VCSEL Spectrum as measured by a ZOOM Spectra throughs a scrambling setup

Comparing these two videos, it can be observed that the output spectrum as observed by the ZOOM Spectra becomes much more stable, with much less intensity variation and peaks displacement over time, when using a multimode scrambling system. This is explained by the fact that scrambling the fiber makes sure that all spatial modes of the different speckles of the MMF always are included into SMF, so no spectral information is lost, which is not necessarily the case when performing a direct coupling.

In addition to this, and to make sure no spectral information was discarded during the coupling process with the scrambling setup, a comparison of the results obtained with the ZOOM Spectra and a high end compact grating spectrometer has been carried out. The main results can be shown below :

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Figure 4 : Measurement of a VCSEL Spectrum with a ZOOM Spectra (blue) and a high resolution compact spectrometer (red)

One can observe from this graph that the spectrum measured with the ZOOM Spectra and the High Resolution compact spectrometer are similar, thus it can be concluded no information is lost during the scrambling process. Moreover thanks to its much higher resolution, the ZOOM Spectra allow the detection of side bands and peaks the high resolution compact spectrometer cannot resolve in a first place..

To conclude this article, we have seen that the high speed measurement of VCSEL spectrum has been made possible by using a combination of a ZOOM Spectra to perform the spectral measurement with great resolution and high speed, and a spatial scrambler SpeckleFreeTM to ensure an efficient optical coupling.

 

 


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