Date of Award

12-2015

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Electrical and Computer Engineering

Advisor

Zhu, Lin

Committee Member

Ballato, John

Committee Member

Dong, Liang

Committee Member

Wang, Pingshan

Abstract

In this thesis, we proposed, fabricated and demonstrated the coherent beam combining of angled-grating broad-area lasers. We have obtained the simultaneous coherent beam combining and single transverse mode operation completely on chip without any external phase control/components. Since the single transverse mode is the key to obtain diffraction-limit beam quality and high brightness, the proposed design is a good candidate for high power and high brightness applications. In the proposed coherently combined laser array, we use the angled-grating broad-area laser as the building block and overlap the adjacent emitters at one facet. The overlapped region becomes a 2D coupling region. And the coherent beam combining is obtained through the Bragg diffraction in these coupling regions. The scalability of the proposed structure is also studied through a simplified zigzag array with the same topographic structure. The random phase difference among emitters in the array is assumed to be Gaussian distribution. And the brightness of the laser array is calculated at different random phase strength in two extreme situations: one is that only adjacent emitters are correlated and the other one is that all the emitters in the array are correlated. In the real zigzag laser array, the power of one emitter can be coupled into multiple neighbouring emitters. The scalability of the proposed structure should be between the two extreme situations. It should be similar to the performance of common cavity laser arrays. The fabricated two coherently combined angled-grating broad-area lasers shows an interference pattern in the far field measurement indicating the two emitters are indeed coherently combined. However, the overall envelope shows double lobes. A further investigation reveals that the double lobes come from the uneven distribution of the injected current due to the lateral current leakage. The uneven current distribution excites the second order Bragg modes resulting in the double lobes in the far field. Therefore, we use ion implantation to increase the resistance outside the metal contact area to obtain a more uniform current distribution. And the laser diodes after ion implantation show a single lobe in the far field envelope with interference patterns within the envelope. The output power of the fabricated lasers is limited by the bad thermal management. There are obvious thermal rollover in the LI curves at high current level. By p-side-down bonding, we bring the active region closer to the heat sink to help with the heat dissipation. Both the single and two combined angled-grating broad-area lasers can deliver over 1W output power without obvious thermal rollover at 180K. They can also lase in room temperature pumped with quasi-CW current source. However the slope efficiency is still low and the threshold is relatively high, which may be due to the high optical and electrical loss induced by the deep III-V dry etching. In order to reduce the etched area for lower optical and electrical loss, we decide to substitute the TBR grating with 2D triangle lattice photonic crystal cavity. There are two advantageous of 2D PC cavity, one is to reduce the surface defect states for less total loss, the other one is to control the longitudinal mode to obtain single wavelength as well as single transverse mode, since structure along the propagation direction is also periodic. The reason for choosing triangle lattice is to easily combine the 2D PC Bragg cavity in the same way we did in the coherent combining of angled-grating broad-area lasers. We solve for the first several photonic bands using MPB and determine the periods along the transverse and propagation directions for design purpose. Since these two periods are geometrically related too, we find discrete tilted angles to satisfied both resonant and geometric requirements. And usually the wavevector along the propagation direction is resonant with a high order grating vector. We fabricated both single and two combined PC Bragg lasers. As expected, the single 2D PC Bragg laser diode presents stable single wavelength optical spectrum without mode hopping during the measurement period. The far field also indicates near diffraction-limited beam quality. However, the combined laser diode shows multiple peaks in the far field profile due to the shallow etching depth. Regrowth wafer is another way to reduce the total loss. In this project, since the grating is wet etched in the cladding layer which is much closer to the quantum well, the grating depth can be pretty shallow. Since we don't have any epitaxy layer growth facilities and experience, the epitaxy layer growth and regrowth process is done by a foundry service. After the quantum well is grown, the wafer is shipped to us and after the grating is etched, we ship them back to the service for regrowth process. Unfortunately, due to the surface cleanness, the wafer after regrowth has a lot of defects in it. All the devices including the broad-area lasers do not lase. Therefore, we couldn't evaluate the performance of the coherently combined lasers using regrowth epitaxy wafer. We also investigate another interesting laser cavity design based on the angled-grating broad-area laser, which is the folded angled-grating broad-area laser. By using the symmetry of the snake-like lasing mode in the angled-grating broad-area laser, the angled-grating broad-area laser can be folded at the center to the other direction without disturbing the lasing mode. The experiment results confirm that with a well design cavity length, the folded cavity has a similar performance to that of the angled-grating broad-area laser. The immediate benefit of this design is to reduce the laser area and increase the yield. More promising is to combine two folded cavity laser to increase the filling factor. It is obvious that in the previous combined laser design, the longer the cavity is, the smaller the filling factor is. By combining two folded cavity lasers, it is possible to increase the filling factor to nearly 1. The fabricated two combined folded cavity lasers show an increased angular distance between two interference fringes in the far field. However, due to the relative low output power, the aperture distance is still larger than designed distance.

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