Over the last two years, our group has reported the first room-temperature continuous-wave (RTCW) fixed wavelength VCSELs operating above 3 microns, in both optically pumped and electrically pumped devices. Our optically pumped 3.3um devices employ one or two wafer-bonded GaAs/AlGaAs mirrors, in conjunction with a type I InGaAsSb/AlInGaAsSb quantum well active region. Our electrically pumped 3.3um devices employ a bottom waferbonded GaAs/AlGaAs mirror, top deposited ZnSe/ThF4 mirror, and type II interband cascade (ICL) active region. These fixed wavelength devices lay a foundation for tunable devices in the spectrally rich 3-5um region. Narrowly tunable devices can use thermal tuning, by variation of pump power (optically pumped devices), bias current (electrically pumped devices), or device temperature (both electrically and optically pumped devices). In this paper, we describe tunable CW optically pumped devices with >4nm of tuning near 3.3um using variation of pump power. CW electrically pumped devices show ~2nm tuning near 3.3um using variation of bias current. These results are a critical first step towards an inexpensive and high-speed methane sensing source. A first generation of MEMS-tunable optically pumped devices has achieved 70nm tuning range near 3.34um.
In recent years, MEMS-tunable VCSELs have emerged as a leading swept source for optical coherence tomography imaging. At the ophthalmic imaging wavelength of 1050nm, optically pumped MEMS-VCSELs (MEMS-oVCSELs) have previously achieved >100nm tuning range and repetition rates approaching 1Mhz, enabling high-resolution and high-speed eye imaging. Electrically pumped MEMS-VCSEL technology (MEMS-eVCSEL) is a critical need for many emerging low-cost high-volume applications, but thus far tuning range has lagged substantially behind optically pumped devices. In this work, we demonstrate 97nm continuous tuning range in a MEMS-eVCSEL operating near 1050nm, and >100nm total tuning range, representing the widest tuning ranges achieved to date, and rivaling the performance of optically pumped devices. Our devices employ a strain-compensated InGaAs/GaAsP gain region disposed on a wideband fully oxidized GaAs/AlxOy back mirror. A deposited top mirror rests on a flexible dielectric membrane separated by a variable airgap from the underlying gain region. Application of voltage between the dielectric membrane and a bottom actuator contact on the top of the gain region creates an electro-static force which pulls the suspended mirror down, contracting the airgap and tuning the device to shorter wavelengths. In this 3-terminal device, the bottom actuator contact doubles as the laser anode. Current injection proceeds from the anode to the cathode at the back of the GaAs substrate through a lithographically defined low-loss current aperture, enabling reproducible aperture size and reproducible single-mode performance. These devices offer promise for many emerging high-volume imaging applications.
Tunable vertical cavity surface emitting lasers (VCSELs) offer a potentially low cost tunable optical source in the 3-5 μm range that will enable commercial spectroscopic sensing of numerous environmentally and industrially important gases including methane, ethane, nitrous oxide, and carbon monoxide. Thus far, achieving room temperature continuous wave (RTCW) VCSEL operation at wavelengths beyond 3 μm has remained an elusive goal. In this paper, we introduce a new device structure that has enabled RTCW VCSEL operation near the methane absorption lines at 3.35 μm. This device structure employs two GaAs/AlGaAs mirrors wafer-bonded to an optically pumped active region comprising compressively strained type-I InGaAsSb quantum wells grown on a GaSb substrate. This substrate is removed in processing, as is one of the GaAs mirror substrates. The VCSEL structure is optically pumped at room temperature with a CW 1550 nm laser through the GaAs substrate, while the emitted 3.3 μm light is captured out of the top of the device. Power and spectrum shape measured as a function of pump power exhibit clear threshold behavior and robust singlemode spectra.