Phase and coherence extraction from a phased vertical cavity laser array

Johnson, Matthew T.; Siriani, Dominic F.; Sulkin, Joshua D.; Choquette, Kent D.

doi:10.1063/1.4736406

Cited by 35 publications

(22 citation statements)

References 27 publications

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“…A simulated far field image is also shown for comparison in Figure 4(c). The image was obtained using the Fraunhoffer approximation to propagate a 7-element hexagonal array comprised of elements with 62 μm beam waists and a pitch of 250 um to the far field, assuming the elements are mutually coherent [11]. The disagreement between the simulated and observed far-field images is attributed to imperfections in the imaging system.…”

Section: Resultsmentioning

confidence: 99%

Demonstration of an in-phase, coherently-coupled 37-element VECSEL array

et al. 2014

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Electrically-injected vertical external cavity surface emitting laser (VECSEL) arrays are an attractive source for lowcost, high-brightness applications. Optical pumping can be used to investigate the emission properties of such devices without undergoing complex device fabrication. The design of such arrays is based on a single VECSEL chip, a 2D lens array, and a flat output coupling dichroic mirror. In this work, we report on the demonstration of an optically pumped, coherently-coupled VECSEL array. The array achieves a maximum total output power of >60 mW and lasing spectrum indicates single-mode operation. Near-field characterization reveals 37 individual lasing elements in a hexagonal array. Far-field measurements show an interference pattern which is consistent with inphase coherent coupling, with >60% of the total output power present in the on-axis central lobe. The physical origin of coherent coupling is attributed to diffractive coupling. The simplicity of the optical cavity design suggests scalability to much larger arrays, making the result of particular interest to the development of low-cost, highbrightness diode sources.

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Section: Resultsmentioning

confidence: 99%

Demonstration of an in-phase, coherently-coupled 37-element VECSEL array

et al. 2014

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“…In this manner the relative phase between the array elements can be varied, and thus the far-field can be electronically steered [7]. Shown in Fig.…”

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Two-dimensional coherently coupled vertical cavity laser arrays

Johnson

Siriani

Fryslie

et al. 2013

2013 IEEE Photonics Society Summer Topical Meeting Series

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Two-dimensional coherently coupled vertical cavity surface emitting laser (VCSEL) arrays have long been pursued for narrow divergence and high brightness applications. We discuss both top and bottom-emitting coherent VCSEL arrays that can operate in the in-phase mode. The key to their operation is strongly coupled leaky-mode operation, which can be achieved using ion implantation to define a periodically arrayed gain regions, and a 2D photonic crystal hole pattern etched into the top facet to control the lateral modes [1,2]. The brightness of VCSEL arrays typically suffer from either incoherence between elements [3] or an out-of-phase far field profile [4], either of which can arise from periphery current injection. Bottom-emitting arrays can circumvent these difficulties by allowing direct current injection into all laser elements. Previous pulsed coherent bottom emitting arrays have employed reflectivity modulation to select the supermode, which necessarily leads to out-of-phase emission [5]. Here we report a simple self-aligned process to demonstrate inphase continuous wave (CW) emission in both top and bottom emitting coherently coupled VCSEL arrays. We also show for top emitting coherently coupled arrays that the phase can be varied by differential current injection over a full range of π which enables beam steering at modulation speeds up to 100 MHz [6].The VCSEL arrays under study are arranged in a 2x1 or 2x2 configuration, where the fabrication procedure has been described in previous work [1,2]. The top emitting arrays emit at 850 nm while the bottom (substrate) emitting arrays emit at 980 nm. The photonic crystal pattern etched into the top mirror limits the number of lasing modes and provides stable index guiding. The defects in the hole pattern create the array elements and are aligned with implant apertures approximately the same size. The latter confines the current to the individual elements, as shown in Fig. 1 for either top emitting or bottom emitting geometry. The top metal contact and uppermost semiconductor layer is etched with a focused ion beam for electrical isolation of each defect cavity to allow preferential current injection. For the bottom emitting arrays, a Au layer with 3 µm thick 7x7 µ m Au squares with a 9 µm pixel pitch is deposited on the high reflectivity DBR mirror. The thick Au squares block subsequent proton implantation, providing self-aligned current confinement to the array elements. The bottom emitting arrays shown here do not have photonic crystal mode control.Shown in Fig. 2 are near and far field profiles from in-phase operation of the arrays. The near field modal pattern of the 2x2 top emitting array in Fig. 2(a) exhibits fringes between the elements [1], which is indicative of the leaky-mode behavior [2]. The current injection path defined by the implantation suppresses the index which enables in-phase operation, as shown in Fig. 2(b). However due to current crowding issues, larger arrays only exhibit out-of-phase operation (with an on-axis far field null) or the inn...

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“…1. The phase shift between elements at each current difference is extracted from fitting the propagated near field to the far field [7]. The field amplitudes and are determined as the square root of the maximum intensity measured in each element.…”

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Beam steering mechanism in phased vertical cavity laser arrays

Johnson¹,

Siriani²,

Choquette³

2013

2013 IEEE Photonics Conference

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Beam steering with optical phased arrays is accomplished by introducing a relative phase shift between array elements [1]. This is typically achieved by altering the refractive index difference between elements, which alters the optical path length of incident radiation traveling through the elements. Beam steering with VCSEL arrays has also been shown to rely on such a refractive index variation [2], which is implemented by adjusting the current injection to the elements. However, if we view the VCSEL array as a waveguide from the active region through the distributed Bragg Reflectors, it can be shown that the index-induced phase shift between elements would only account for less than 1/100 th of the observed phase shift [2]. It is thus evident that phased VCSEL arrays rely on a fundamentally different phase-shifting mechanism. To investigate this, we turn to coupled mode theory, which has previously been applied to the dynamics of coupled edge emitting laser arrays and phase tuning in injection-locked VCSELs [3,4].The dynamic coupled mode equations for semiconductor laser arrays can be normalized to yield [3] 1 , 2where is the coupling strength between elements, the photon lifetime, the constant coupling phase (0 for in-phase arrays, for out-of-phase), the linewidth enhancement factor [5], the angular frequency, the normalized field amplitude, the phase, and the normalized excess carrier density in the laser. For a two-element array where = 1(2) for the left (right) element and , the phase shift between elements is found to correspond with the native cavity resonance of each element, per 3 , 4where coupled ω is the angular frequency of the coupled emission.Coherently coupled leaky mode VCSEL arrays with and are examined where the current injection into each element is varied [6]. The 2x1 arrays are coherently coupled together when the difference between their native resonance frequencies, , falls within a certain locking range which corresponds to a range of varying injection current. Outside of this range, the elements are assumed to operate independently at their native resonances, which must be further examined. The far field profiles of these arrays are shown at varying current differences between the elements in Fig. 1. The phase shift between elements at each current difference is extracted from fitting the propagated near field to the far field [7]. The field amplitudes and are determined as the square root of the maximum intensity measured in each element.As can be seen from the inset of Fig. 1(b), for the array. In Fig. 2 we show the phase difference retrieved from experiment and the lasing emission wavelength. The coupling strength is approximated from Eqn. 4 as , where is the maximum detuning between resonant frequencies, outside of which spectral splitting is observed. This value is obtained from as shown in Fig. † Currently at MIT Lincoln Laboratory, Lexington, MA 02420 ( ) ( ) [ ] 1 1 1 1 ( ) sin sin m m m p m m m m m m X X X d t Z X dt κτ φ φ ψ φ φ ψ + + − − = − − + − − + ( ) ( ) ( ) ( ) [ ] ( ) ...

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Phase and coherence extraction from a phased vertical cavity laser array

Cited by 35 publications

References 27 publications

Demonstration of an in-phase, coherently-coupled 37-element VECSEL array

Demonstration of an in-phase, coherently-coupled 37-element VECSEL array

Two-dimensional coherently coupled vertical cavity laser arrays

Beam steering mechanism in phased vertical cavity laser arrays

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