Realization of a Semiconductor-Based Cavity Soliton Laser

The realization of a cavity soliton laser using a vertical-cavity surface-emitting semiconductor gain structure coupled to an external cavity with a frequency-selective element is reported. All-optical control of bistable solitonic emission states representing small microlasers is demonstrated by injection of an external beam. The control scheme is phase insensitive and hence expected to be robust for all-optical processing applications. The mobility of these structures is also demonstrated.

Figure 1

Experimental apparatus. The external cavity is 616 mm long, including an 8 mm aspherical lens used as a collimator and a 300 mm lens: it is thus self-imaging, projecting a $37.5\times$ image of the VCSEL onto a Littrow-mounted holographic diffraction grating ($1800\mathrm{\text{lines}}/\mathrm{mm}$). Beam sampler BS1 couples a writing beam into the cavity, while BS2 couples out part of the beam for detection. Half-wave plates (HWP) are used to match the principal axes of the VCSEL, the intracavity beam splitters, and the grating. The strong polarization dependence of the grating’s diffraction efficiency ensures that the intracavity field has a well-defined linear polarization. Hence the HWPs can also be used to control the coupling efficiency of the writing beam and of the detection arm to the external cavity. For some measurements a comb filter is added in the reimaged near field close to the grating. Its slits are orthogonal to the grating lines.

Figure 2

Power versus current for a single spot. In this case the comb filter (explained below) was inserted in order to reduce the optical background of the spot and hence obtain a cleaner hysteresis. The panels on the right show, from top to bottom, the near-field intensity distribution around the spot, a transverse intensity profile through the center of the near-field distribution, and the far-field intensity distribution.

Figure 3

Near field intensity distributions showing the successive switching on and off of two spots with an injected incoherent field (brightest spots, indicated by arrows). Dark areas correspond to high intensities. The WB is derived from a tunable laser source, with wavelength tuned in the vicinity of the VCSEL cavity resonance. It is focused onto the VCSEL with a $12\mu \mathrm{m}$ spot diameter (FWHM) and a power in the mW range. Two sites where spontaneous spots could be observed were selected for WB injection, and the VCSEL was biased within their bistability range. (a) Both spots are off, (b) injection of WB, (c) one spot is switched on and remains after the WB is blocked, (d) injection of WB at a second location, (e) second spot remains on, (f) WB injected beside second spot, (g) second spot switched off and does not reappear (first spot unaffected), (h) injection of writing beam to switch off first spot, and (i) both spots remain off.

Figure 4

Near field intensity distributions displaying the effect of a comb filter placed in the external cavity at the reimaged near-field position. Dark areas correspond to high intensities. This filter allows feedback only on several horizontal stripes with width $16\mu \mathrm{m}$ at the VCSEL, spaced by the same distance. From (a) to (d) the comb filter is moved downward. A line is added for reference, clearly showing that CS positions are shifted between panels. For example, the rightmost CS close to the line in (a) shifts quasicontinuously by about $18\mu \mathrm{m}$ between (a) and (d). Some CSs appear and disappear spontaneously, as a result of local inhomogeneities.