Abstract
A distinctive method is used to achieve transverse-mode control by building and integrating a tunable liquid crystal (LC) mode filter on top of traditional vertical-cavity surface-emitting lasers (VCSELs). An LC cell is fabricated by injecting a type of nematic LC material (E7) in an annular groove patterned by SiO2. By electrically tuning the refractive index of the LC, a spatially dependent reflectivity profile can be achieved and optimized, which directly influences the threshold modal gain of each transverse mode provided by oxide-confined VCSELs. The flexible and accurate control performance of the LC mode filter structure have been demonstrated by the simulation results and our analysis. This design is a completely novel method to realize VCSELs with an accurate and real-time controllable transverse-mode, and it will probably play a significant role in the future.
Highlights
Over the past 40 years, vertical-cavity surface-emitting lasers (VCSELs) have experienced tremendous development and improvement.1–4 Due to many unique advantages such as low energy consumption, low fabrication cost, perfect circular light spot, and manufacturable in 2D arrays,5–7 they have increasingly been researched and applied in various fields, and they have been becoming an indispensable element around us as widespread mobile phone parts
The 940 nm VCSEL epitaxial structure used in this work can be grown in molecular beam epitaxy (MBE), which consists of an n-doped bottom and p-doped top distributed Bragg reflector (DBR), which is alternately stacked by Al0.9Ga0.1As and Al0.12Ga0.88As in 30 pairs and 19.5 pairs, respectively
To illustrate the effect of mode control and real-time adjustment of the liquid crystal (LC) cell structure, the threshold modal gain (TMG) is calculated as functions scitation.org/journal/adv of the thickness and refractive index of the LC for y-polarization modes, which is tuned by the tuning voltage Vt, and the results have been plotted in Fig. 6(a), from which we can see that the TMGs at different thicknesses from 3.53 μm to 3.63 μm can be adjusted by electrically tuning the refractive index
Summary
Over the past 40 years, vertical-cavity surface-emitting lasers (VCSELs) have experienced tremendous development and improvement. Due to many unique advantages such as low energy consumption, low fabrication cost, perfect circular light spot, and manufacturable in 2D arrays, they have increasingly been researched and applied in various fields, and they have been becoming an indispensable element around us as widespread mobile phone parts. Metal apertures, ion implantation, extended cavity, and impurity-induced disordering of the top distributed Bragg reflector (DBR) have successfully been achieved in lasing-mode control. For all these methods, their modefiltering function is largely confined by the processing precision (including regrowth or etching processing), and the structures cannot be fine-adjusted once processing has been completed, resulting in a very low yield, which is one of the factors that limit their practical mass production. Unlike the traditional mode selective design mentioned above, this LC mode filter structure can be adjusted accurately to achieve modeselection and mode-control by controlling the bias voltage applied on the LC cell rather than only relying on the high precision of device processing
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