A sensitive structure with built-in T-shaped beams was studied in the paper to achieve high fill-factor and high sensitivity for Micro-Optical-Electro-Mechanical-Systems (MOEMS) application. The silicon proof mass in the structure was supported by four identical T-shaped beams, which were distributed symmetrically and orthogonally in the plane to suppress the in-plane cross coupling. In particular, the T-shaped beam was composed of three parts: stress releasing structure, cantilever and flexible linking structure. The stress releasing structure was used to avoid torsion or warpage caused by residual stress and improve the sensitivity at the same time. The mechanical properties were studied systematically by finite element simulation. The stiffness in z-axis direction was much lower than the in-plane stiffness of the structure, indicating high z-axis sensitivity and small cross coupling error. The reason for high sensitivity of the sensitive structures was fully illustrated by comparing the displacement responses for different beams. The simulation results indicated that the sensitivity was improved more than twice because the stress releasing structure and flexible linking structure reduced the axial stress caused by deflection. Finally, the optical performances were also evaluated in terms of bandwidth and tuning range when used for MEMS Fabry Perot Optical Tunable Filter. The wavelength tuning range achieved about 1.8μm in long-wave infrared waveband by controlling the applied voltage.
Transmission characteristics of Fabry-Perot (F-P) filter based on silicon substrate with different transmissivity of high reflection (HR) coatings, incident angle, and interference orders are investigated. The results show that the transmissivity of HR coatings has great effect on full width at half maximum (FWHM) of transmission spectrum, the FWHM of F-P filter reduced from 209 nm to 3.4 nm with the reflectivity of HR coatings increased from 84.7% to 99.6%. The peak wavelength shifts from 1546.3 nm at 0° to 1542.6 nm at 5°, indicating that the FWHM of transmission spectrum broadens as the incident angle increases. The 1st, 2nd, 3rd, and 4th order interference are 3.4 nm, 2.3 nm, 1.8 nm, and 1.5 nm, respectively. Thus, in the applications tuning in a narrow wavelength range, F-P filter can be designed to operate in high-order mode to achieve a narrow transmission spectrum.
High spatial coherence can maintain the beam stability of the laser after a long distance. However, it limits the applications in laser display including projection and imaging system, because the high coherence of laser diodes cause the artifacts such as speckle. In this work, we design a novel 6xx nm chaotic cavity laser diode, which consists of a Dshaped section used to achieve a large number of independent spatial modes thus reduce coherence and a stripe area to improve power. The radius of the D-shaped cavity is 500 μm and the length of stripe is 1000 μm. The red laser based on GaAs substrate is fabricated by standard photolithography and reactive ion etching process. To obtain an enough optical confinement by effective refractive index step, the etching depth exceeds the active region. The high-power chaotic cavity low-spatial coherence electrically pumped semiconductor laser is first realized with the wavelength around 630 nm. The spectrum width of 15 nm at full width at half maximum (FWHM) and output power of 300 mW is obtained under pulse operation. The speckle contrast is measured to be 5%, showing great potential of reducing speckle from the source directly for laser display.
A tilted wave (TW) laser based on the quasi-periodicity photonic crystal (QPC) structure is studied to improve the maximum power and wall-plug efficiency of laser diodes. In this work, we first use the longitudinal QPC structure instead of the thick passive waveguide compared with previously reported. In order to localize the first order mode, we designed and optimized the longitudinal QPC structure. The optical confinement factor (OCF) of the fundamental mode is only onesixth of the first order mode. In the experiment, a continuous wave power of 12.8 W at 980 nm is achieved with a peak power conversion efficiency of 57.2%. The laser emitted two nearly symmetric narrow vertical beams in far field pattern, which has a full width at half maximum (FWHM) of 7.6° each.
Ridge-waveguide (RW) lasers based on photonic crystal structure were fabricated and measured. We investigated the effect of residual layer thickness (corresponding to etching depth) and ridge width on electro-optical characteristics of RW lasers. For deep-etching RW lasers, although lateral beam quality factor M2 is better than that of shallow-etching RW lasers, the other characteristics such as output power are much less than that of shallow-etching RW lasers. The calculating results indicate that RW lasers with ridge width w ≥ 8 μm will operate in mixing mode. The experimentally results of various ridge width RW lasers show that RW laser with 7 μm ridge operated in single mode over the whole measurement range and RW laser with 8 μm ridge change from single-mode operation to mixing-mode operation with the increasing of driving current. The device with 7-μm-wide ridge and 3-mm-long cavity obtain 2 W single-transverse-mode optical power and 59% maximum power conversion efficiency. The lateral beam quality factors M2 values are less than 1.7 over the whole measuring range.
High power and high beam quality laser sources are required in numerous applications such as nonlinear frequency conversion, optical pumping of solid-state and fiber lasers, material processing and others. Tapered lasers can provide a high output power while keeping a high beam quality. However, the conventional tapered lasers suffer from a large vertical beam divergence. We have demonstrated 2-mm long tapered lasers with photonic crystal structures. A high beam quality and a narrow vertical divergence are achieved.
In this paper, we optimized the photonic crystal structure and fabricated a 4-mm long tapered laser to further increase the output power and the wall-plug efficiency. Compared with our precious wafer, the optimized structure has a lower doping level to reduce the internal loss. The period of the photonic crystal structure and the thickness of the upper cladding are also reduced. The device has a 1-mm long ridge-waveguide section and a 3-mm long tapered section. The taper angle is 4°. An output power of 7.3 W is achieved with a peak wall-plug efficiency of 46% in continuous-wave mode. The threshold current is around 500 mA and the slope efficiency is 0.93 W/A. In pulsed mode, the output power is 15.6 W and the maximum wall-plug efficiency is 48.1%. The far-field divergence with full width at half maximum is 6.3° for the lateral direction at 3 A. The vertical far-field beam divergence is around 11° at different injection levels. High beam qualities are demonstrated by beam quality factor M2 of 1.52 for the lateral direction and 1.54 for the vertical direction.
High efficiency 980 nm longitudinal photonic band crystal (PBC) edge emitting laser diodes are designed and fabricated. The calculated results show that eight periods of Al0.1Ga0.9As and Al0.25Ga0.75As layer pairs can reduce the vertical far field divergence to 10.6° full width at half maximum (FWHM). The broad area (BA) lasers show a very high internal quantum efficiency ηi of 98% and low internal loss αi of 1.92 cm-1. Ridge waveguide (RW) lasers with 3 mm cavity length and 5um strip width provide 430 mW stable single transverse mode output at 500 mA injection current with power conversion efficiency (PCE) of 47% under continuous wave (CW) mode. A maximum PCE of 50% is obtained at the 300 mA injection current. A very low vertical far field divergence of 9.4° is obtained at 100 mA injection. At 500 mA injection, the vertical far field divergence increases to 11°, the beam quality factors M2 values are 1.707 in vertical direction and 1.769 in lateral direction.
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