By using an optical feedback, what is the possibility of reducing or enhancing the non-linearity properties of VCSEL with modulation signal?
The nonlinearity doesn't necessarily means that the signal undergoes instability. However if one tries to have unstable system through the optical nonlinear medium, he/she can introduce feedback to the system which can be in the form of internal or external feedback. This will add a term to nonlinear Schrodinger equation which can be interpreted as strengthen of nonlinearity.
I suspect it can go either way, depending on feedback strength, e.g., 0 dB down to -50 dB, and also the phase of the feedback. Similar things happen to RIN (laser noise). See K Petermann, and other papers from the 1980s, on laser feedback impact on RIN.
We do use external optical cavities in VCSELs and Fabry Perot for measurements. Small levels of feedback (roughly upp to 10-4 of the emitted power reflected back in the cavity) yield cosinusoidal or quasi sinusoidal amplitude modulation states. If feedback rises you'll start with mode hopping, and if it rises further then you'll get chaos and coherence collapse behaviour. You can find a brief description in the introduction of this PhD dissertation (it's in English)
http://www.tdx.cat/handle/10803/134357
or in some of the papers on self-mixing interferometry or optical feedback interferometry available elsewhere.
There are many factors to analyze, however, out of your question. Which nonlinearity are you referring to? The nonlinear effects in the media inside the cavity, the nonlinear response to modulation current, or...
Thanks to all, dear Santiago Royo, what I observed is the nonlinearity of VCSEL when applied external optical feedback ,by means of observing the capturing the spectrum of the signal in the electrical domain
It is a very well described effect which as I commented depends strongly in the level of feedback. From the theoretical point of view look for the seminal paper of Lang&Kobayashi.
Feedback from an external reflector coherently adds to reflectivity of the internal cavity. If the external cavity contains many wavelengths, then current-induced changes in the laser frequency will change the phase of the external reflection with respect to the internal optical signal. This can result in a sinusoidal change in laser threshold with increasing drive current, resulting in a nonlinear current to light transfer function. A similar effect is seen with early fabry-perot semiconductor lasers when coupled to short optical fibre without an intermediate optical isolator. I am not sufficiently familar with VCELs to know if the same will occurr.
Thank you Richard, I would be very appreciate if you could please refer to a reference
Salam, try:
Lang, R.; Kobayashi, K., "External optical feedback effects on semiconductor injection laser properties," Quantum Electronics, IEEE Journal of , vol.16, no.3, pp.347,355, Mar 1980
doi: 10.1109/JQE.1980.1070479
URL: http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=1070479&isnumber=23007
Also:
Klaus Petermann; External optical feedback phenomena in semiconductor lasers. Proc. SPIE 2450, Broadband Networks: Strategies and Technologies, 121 (February 17, 1995); doi:10.1117/12.201293.
The longer the external cavity, the smaller the change in laser frequency required to bring internal and external cavities in-phase. Back in 1975 we observed staircase shaped light-current profiles when coupling an FP laser to a 2.5cm fibre tail (which was part of a demountable fibre connector).
Significantly longer external cavities lead to high levels of intensity noise, and broadened linewidth. However we have used the rayleigh scattering feedback ( from monomode fibres between 100m and 1 km in length) to achieve very high coherence: “A Practical 1.3 microns Semiconductor Source with significantly better short term coherence than a gas laser”, R.E.Epworth, D.F.Smith, S. Wright, Proc ECOC 10, Stuttgart, 1984.
Increasing the operating current will decrease the nonlinear effect, otherwise increase the nonlinear effect.
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