Spectrally resolved imaging of the transverse modes in multimode VCSELs

Abstract

Vertical-cavity surface-emitting lasers (VCSELs) enable a range of applications such as data transmission, trace sensing, atomic clocks, and optical mice. For many of these applications, the output power and beam quality are both critical (i.e. high output power with good beam quality is desired). Multi-mode VCSELs offer much higher power than single-mode devices, but this comes at the expense of lower beam quality. Directly observing the resolved mode structure of multi-mode VCSELs would enable engineers to better understand the underlying physics and help them to develop multi-mode devices with improved beam quality. In this work, a low-cost, high-resolution (<3 pm) Echelle grating spectrometer system is used to map the two-dimensional VCSEL near-field emission profile. The system spectrally disperses the VCSEL beam and images it with high magnification onto a CMOS camera. The narrow spectral content of each LP mode allows direct observation of the modal content of the VCSEL. MOTIVATIONSemiconductor lasers emit light perpendicular to their surface while VCSELs are built with layers of distributed Bragg reflectors (DBRs) and a central active region containing multiple quantum wells. The photons are emitted by pumping current into a ring electrode in the active region between oppositely doped DBR layers, forming a p-n junction diode. While the range of possible emission wavelengths is limited by the materials used in the VCSEL design, the exact lasing frequency is selected by the design of the resonant cavity (in all three dimensions). VCSELs designed with a larger mode areas result in higher power, however beam quality is degraded due to multi-mode operation in both the lateral and vertical direction.Directly observing the resolved 2D mode structure of multi-mode VCSELs would help engineers develop a better understanding of the underlying physics in order to design multi-mode devices with improved beam quality. The typical near-field emission profile of a VCSEL is a 2D image which shows the overall intensity pattern arising from the interaction of all lasing modes, each operating at a slightly different wavelength. In a manner similar to the 1D case for edge-emitting lasers, the observed near-field mode profile is simply the summation of the individual eigenmodes of the 2D VCSEL cavity. Figure 1. Example near-field image profile of a multimode VCSEL Vertical-Cavity Surface-Emitting Lasers XIX, edited by Chun Lei, Kent D. Choquette, Proc. of SPIE Vol. 9381, 93810L · © 2015 SPIE · CCC code: 0277-786X/15/$18 ·

Nonlinear polarization and transverse mode dynamics of a broad-area VCSEL in continuous wave operation

“…The polarizer was aligned with the u and v polarization axes. Spatio-spectral measurements of BA-VCSELs using an imaging spectrometer [52,[66][67][68] or a state tomography technique [56] have been reported previously.…”