Phase-locked laser arrays through global antenna mutual coupling

Journal name:
Nature Photonics
Volume:
10,
Pages:
541–546
Year published:
DOI:
doi:10.1038/nphoton.2016.104
Received
Accepted
Published online

Abstract

Phase locking of an array of lasers is a highly effective method in beam shaping because it increases the output power and reduces the lasing threshold. Here, we show a conceptually novel phase-locking mechanism based on ‘antenna mutual coupling’ in which laser elements interact through far-field radiations with definite phase relations. This allows a long-range global coupling among the array elements to achieve a robust phase locking in two-dimensional laser arrays. The scheme is ideal for lasers with a deep subwavelength confined cavity, such as nanolasers, whose divergent beam patterns could be used to achieve a strong coupling among the elements in the array. We demonstrated experimentally such a scheme based on subwavelength short-cavity surface-emitting lasers at terahertz frequencies. More than 37 laser elements that span over ∼8 λo were phase locked to each other, and delivered up to 6.5 mW (in a pulsed operation) single-mode radiation at ∼3 THz, with a maximum 450 mW A–1 slope efficiency and a near-diffraction-limited beam divergence.

At a glance

Figures

  1. Mutual admittance and electric-field distribution of the QCL.
    Figure 1: Mutual admittance and electric-field distribution of the QCL.

    a,b, Schematic for calculating two slot antennas on an infinite ground plane (assuming the direction normal to the slot antenna facet is along the y direction and one of the slot antenna is placed at the origin) (a) and two three-section surface-emitting DFB lasers (top view) (b). θ ≡ arctan (y/x). c, The electric-field distribution inside the three-section second-order DFB laser when operated in the surface-emitting mode. The relative polarity of each facet is also shown as (+1, +1, +1, −1, −1, −1). The change in polarity sign is the desired outcome of the centre π shift, which is introduced to obtain single-lobe beam patterns from the surface-emitting laser28. The longer section is 32 µm in length (∼λS) with a 4.5 µm gap size. The length of the short centre cavity is 16 µm (∼λS/2) and the ridge width is 15 µm. d, Mutual admittance GM between two slot antennas calculated using equation (1). e, Mutual admittance Gsum between two three-section DFB lasers by summing the mutual admittances between slots. f,g, The sum of all mutual admittances, Ggrid, from other elements in the array under a Type I grid (f), and the Ggrid for a Type II grid (g). Only the nearest-neighbour contributions are included in this simulation. The width of the slot is W = 15 µm with a wavelength of λ0 = 100 µm. The element is the three-section DFB laser shown in c. The xy coordinates are normalized to the free-space wavelength λ0. Certain sets of grid sizes are marked with dotted lines as a visual aid.

  2. Magnetic (H) fields inside the laser cavity and far-field beam patterns from 3D FEM simulations.
    Figure 2: Magnetic (H) fields inside the laser cavity and far-field beam patterns from 3D FEM simulations.

    ac, A single three-section second-order DFB laser (a), a nine-element array of three-section DFB lasers (b) and a 16-element array of DFB lasers (c) under a Type I grid with x block size = 85 µm and y block size = 175 µm. The single-lobe beam divergence reduces with an increasing number of elements in the array. d,e, Simulated magnetic fields inside the laser cavity and far-field beam patterns for the high-order longitudinal mode of a single three-section DFB laser (d) and a nine-element array of DFB lasers (e). Although the beam divergence also reduces with an increasing number of elements in the array, a beam pattern of multiple lobes is clearly shown.

  3. The laser arrays.
    Figure 3: The laser arrays.

    a, Schematic of coupled laser arrays with the three-section DFB laser as the building element. The outer dimension is 2 mm and the diameter of the inner circle is ∼1.7 mm. The laser array is separated into four subparts for biasing in each quadrant individually. The bonding pads are also shown. The number marked on each pad indicates the total number of the elements electrically connected by each bonding pad. b, Schematic of a three-section surface-emitting second-order DFB laser. cf, SEM pictures of the antenna mutual-coupled laser array arranged in a Type II grid. c, The overall view of the laser array. The vertical cut in the centre on the biasing rims that separates the laser array into four quadrants is clearly seen at this angle. d, Close-up view of the grid configuration. e, Single-element three-section DFB laser. The laser consists of three segments: one short half-wavelength centre cavity and two one-wavelength cavities on both sides. The current is provided to the laser through the metal contact on the air-bridge structure. f, Close-up view of one end of the air-bridge structure. The dielectric with a dark shade beneath the metal contact is an electrical insulation layer formed by SiO2.

  4. The four quadrants of the laser array.
    Figure 4: The four quadrants of the laser array.

    a, Pulsed IV curves (top), LV curves (bottom) and lasing spectra (pulsed; inset in top panel) from different quadrants on laser array Chip No. 27. The measurement from a single-element three-section DFB laser is also shown after scaling up the power axis. From the LV curves, a clear change in the lasing thresholds between the single-element laser and the phased-locked laser array can be observed. Current flowing through a single-element device is labelled on the axis on the right-hand side (top), from which one can infer the total current by multiplying the current by the number of array elements. All the quadrants show single-mode emission when C1, C2 and C3 lase at the same frequency. The emission frequency of C4 is slightly red shifted from those of the other three quadrants, which probably results from fabrication variation or post-fabrication contamination. The resolution of the FTIR spectrometer is 0.125 cm−1. The laser array in C1 delivers 6.5 mW peak power (black dashed line) with a maximum ∼450 mW A–1 slope efficiency (bottom). b, Terahertz images of the emission from the laser array when different quadrants are biased. The images show narrow single-lobe symmetric terahertz emissions from the laser arrays. The lasers are only biased slightly above the lasing threshold to prevent the terahertz emissions from saturating the camera, so that the outline of the laser array chip could be visible in the images. c, Far-field beam patterns from different quadrants of the phase-locked laser arrays measured using a pyroelectric detector located ∼20 cm away from the laser array and mechanically scanned over ±30° in both horizontal and vertical directions.

  5. Type I and Type II grids.
    Figure 5: Type I and Type II grids.

    a,b, Beam patterns (taken by the terahertz camera) (a) and emission spectrum (b) for the laser array arranged in a Type I grid. c,d, The same measurements on another coupled laser array arranged in a Type II grid. Although three-section DFB lasers with identical dimensions are used as the building elements in both arrays, the two laser arrays clearly lase in different spectral and spatial modes.

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Author information

Affiliations

  1. Department of Electrical Engineering and Computer Science and Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA

    • Tsung-Yu Kao &
    • Qing Hu
  2. LongWave Photonics LLC, Mountain View, California 94043, USA

    • Tsung-Yu Kao
  3. Sandia National Laboratories, Center of Integrated Nanotechnologies, MS 1303, Albuquerque, New Mexico 87185-130, USA

    • John L. Reno

Contributions

T.-Y.K. conceived the strategy, designed and fabricated the antenna mutual coupled laser arrays and performed the measurements and analysis, and J.L.R. provided the material growth. All the work was done under the supervision of Q.H.

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The authors declare no competing financial interests.

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