Solitons Enable Semiconductor Microlasers
As water ripples on a pond, light waves usually spread out due to diffraction, if they are not confined by some means. Hence, typical lasers contain curved mirrors to refocus the beam and to create a “stable” cavity. It is also well known that nonlinearities can provide guiding. In that case the light field sustains its own waveguide leading to a soliton.
Recently, researchers from the University of Strathclyde (Yann Tanguy, Thorsten Ackemann, Willie J. Firth) and from Ulm Photonics (R. Jäger) married the curious properties of soliton waves with a major semiconductor laser technology to create a new type of semiconductor laser that can be switched on and off using light pulses. This cavity soliton laser can be applied in all-optical telecoms systems, in which data are switched and routed without the need to convert light pulses into electrical signals and back again.
It has also interesting fundamental aspects because the emerging microlasers have the freedom to choose frequency, phase and polarization in contrast to other optical solitons where these degrees of freedom are usually imprinted by the driving beam. The achievement is published in Physical Review Letters (PRL 100, 013907, 2008) . Fig. 1 shows some of the authors together with the system and some solitons. Yann Tanguy left the group by now but Neal Radwell is working on the further development of the scheme.
Fig. 1: Willie, Thorsten and Neal working on the advancement of the cavity soliton laser. The setup for the external cavity is in the foreground with the mount for the VCSEL on the right (on the heat sink). The monitor shows five solitonic mircrolasers present in the aperture.
The device is based on a vertical-cavity surface-emitting laser (VCSEL), which is used in a wide range of applications including optical datacoms. The VCSEL was built by R. Jäger from ULM Photonics in the framework of the European project FunFACS(Fundamentals, Functionalities and Applications of Cavity Solitons). These devices are magnificent pieces of engineering where nearly hundred of different semiconductor layers with widths in the nanometer to some hundreds of nanometers range (1 nm = 0.000000001 m) are grown using a process called molecular beam epitaxy (MBE).
The VCSEL is driven with a rather small current such that it is amplifying but not yet above threshold, if operating isolated. The device is coupled to an external cavity containing a diffraction grating as a frequency-selective element. The frequency experiencing optimal feedback is deliberately offset slightly from the frequency of the free-running VCSEL. Then a pulse from an external laser is fired at a small spot (12 µm diameter) within the 200 µm aperture of the VCSEL. This changes the index of refraction at that spot and aligns the resonance frequency of the VCSEL locally with the one of the external cavity modes causing a bright spot of light – the soliton - to form in that region.
This soliton represents a small microlaser. It can then be switched off by firing a second laser pulse at the region, which perturbs the refraction index profile in such a way to destroy the soliton. Hence, this microlaser is bistable similar to electronic flip-flops. Similarly, additional microlasers can be set and reset with the external control beam, (in principle) anywhere in the active area, leading to an “ensemble” of small microlasers. Due to the self-localization there is no need for micro-fabrication of individual emitters. The switching sequence is illustrated in Fig. 2.
Fig. 2: Intensity distribution within the active area of the VCSEL. The independent switch-on and switch-off of two microlasers with an external control beam (arrow) is demonstrated. The individual microlasers have a size of about 10 µm. There are also other solitons as well as background states which are attributed to the finite bandwidth of the feedback and inhomogeneities.
As a result, we have optically controllable microlasers, being an example of a major thrust of the field of photonics, the control of light by light.
In the first 2008 issue, a possible application of cavity solitons was starring on the title page of Applied Physics Letters. A collaboration of French, Italian, German and UK researchers from the FunFACS project were reporting on a cavity soliton device used to delay the propagation of light pulses (Appl. Phys. Lett. 92, 011101, 2008). All-optical delay lines and “slow light” are a hot topic in photonics because of fundamental issues as well as of the possible use as all-optical buffers in future high-speed photonic networks.
Currently, the Strathclyde group is working on the advancement of the device by miniaturizing it and achieving the delay functionality in a cavity soliton laser.
"Realization of a Semiconductor-Based Cavity Soliton Laser"
Y. Tanguy, T. Ackemann, W. J. Firth, R. Jäger,
Phys. Rev. Lett. 100, 013907 (2008), Abstract Link.
"All-optical delay line using semiconductor cavity solitons"
F. Pedaci, S. Barland, E. Caboche, P. Genevet, M. Giudici, J. R. Tredicce, T. Ackemann, A. J. Scroggie, W. J. Firth, G.-L. Oppo, G. Tissoni,
Appl. Phys. Lett. 92, 011101 (2008), Abstract Link.
-- FunFACS project
-- Nonlinear Photonics at Strathclyde
-- Computational Nonlinear Optics and Quantum Optics at Strathclyde
-- Ulm Photonics
-- Coverage at Physicsworld.com
-- Popular account of VCSELs