The spatial coherence and the optical phase distribution across a two-dimensional ͑2D͒ photonic crystal implemented with coupled arrays of vertical cavity surface emitting lasers ͑VCSELs͒ are experimentally characterized. This is achieved by performing Young's interference experiments between pairs of array elements using a spatial light modulator arrangement. In contrast to far-field measurements that provide information only on the global spatial coherence, this approach can yield full mapping of the complex degree of spatial coherence. Examples of such analysis are presented for nominally uniform one-dimensional and 2D arrays of coupled VCSELs and possible mechanisms of the observed coherence degradation are discussed. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2431474͔The output power of single-mode vertical cavity surface emitting lasers ͑VCSELs͒ is typically limited to a few milliwatts because of their small aperture area required for higher-order spatial mode suppression.1 Increasing the aperture size leads to poor selection between spatial modes and, above a certain size, results in uncontrolled filamentation, which limits the output power and degrades the spatial coherence and beam quality of the device. The problem of filamentation can be overcome by coupling a large number of single-mode VCSEL, forming two-dimensional ͑2D͒ arrays of phase-locked emitters. [2][3][4][5] In addition, these arrays exhibit a high degree of spatial coherence, as inferred from their nearly diffraction limited, four-lobed far-field patterns, which indicate that they oscillate predominantly at the lowest loss out-of-phase supermode. 2-5Evaluating the features of spatial coherence of coupled-VCSEL arrays is important not only for optimizing their coupling but also for developing functionalities that rely on such coherence. An example of such application is beam steering, where control over the mutual phase of the emitters is necessary.The spatial coherence in phase-locked VCSEL arrays has traditionally been evaluated using measurements of their far-field patterns 2,3 complemented by model calculations of their supermodes. 4,5 However, this approach yields information only on the global coherence properties of the array, in particular, the deviation of the beam pattern from the expected diffraction limited distribution. Moreover, such analysis generally cannot give direct indications on the mechanisms of coherence degradation. Spectral analysis of such arrays, e.g., using spectrally resolved far-field patterns or spatially resolved emission spectra, could give more indications on spatial coherence, but is difficult due to the small spectral splitting of the supermodes in large arrays.A more complete evaluation of the spatial coherence across a VCSEL array would be to measure the complex degree of spatial coherence ␥͑x , y ; xЈ , yЈ͒ between pairs of points in the array plane ͑x , y͒. This can be accomplished by performing Young's interference experiments, in which the interference pattern corresponding to two selected...

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We present a two-dimensional (2D) spatial lock-in amplifier that provides a contrast ratio of more than 10,000:1 for transmitted and blocked intensity patterns using a conventional liquid-crystal spatial light modulator. The device is based on spatial-domain modulation-demodulation of intensity patterns under coherent imaging conditions. The operation of the 2D lock-in amplifier is illustrated by implementing Young's double-slit arrangement for measurements of the mutual coherence between individual emitters of a 2D phase-coupled array of vertical cavity surface emitting lasers.

Formation of bonding and antibonding states of two coupled photonic crystal heterostructure islands implemented with arrays of vertical-cavity surface-emitting lasers is studied theoretically and experimentally. Coupling of the photonic envelope wave functions confined to each island is brought about by tunneling across the heterobarrier separating the islands. Numerical simulations predict the bonding state to have the lowest modal losses. The experimental observations of lasing supermodes confirm this prediction, showing the island coupling in the bonding state of the coupled envelope functions.

The authors demonstrate electrically controlled mode switching and beam steering in separate-contact photonic crystal heterostructures implemented with two-dimensional arrays of coupled vertical-cavity surface-emitting lasers. Two weakly coupled, lasing photonic crystal domains are switched in and out of mutual coherence by controlling the injected currents. Switching of the transverse lasing mode is accompanied by corresponding variations in the far-field pattern and the emission wavelength. Proper adjustment of the driving currents also allows steering of the beam of the ensemble of coupled lasers.