סמינר: Electro-Optics and Microelectronics Seminar
Characteristics of Vertical Cavity Surface Emitting lasers used in Chip Scale Atomic Clocks
Chip scale atomic clocks (CSAC) rely on a specific atomic resonance – CPT – which requires two FM modulated sidebands at a spectral distance of 6.8 [Ghz] centered at 795 [nm]. The only way to obtain this spectrum in the small volume of the chip scale atomic clock is to directly modulate a Vertical Cavity Surface Emitting Laser (VCSEL) at 3.4 [Ghz].
My research addresses the properties of these special VCSELs and includes the following parts.
- Design and construct a measuring system able to measure all the different important parameters of VCSELs while keeping the system user friendly and the measurements repeatable and reliable.
- Fully characterize VCSELs made by VIXAR, the leading supplier of VCSELs used in chip scale atomic clocks. The VIXAR lasers serve as benchmarks for comparison with VCSELs that were designed and fabricated at Technion.
- Fully characterize VCSELs produced at Technion. These turned out to be superior to their VIXAR counter parts.
In my seminar I will describe the system I constructed, followed by detailed measurements of both VIXAR and Technion VCSELs. These include:
Static measurements: I-V and the calculated characteristics, L-I and the calculated , curves, as well as wavelength dependencies on temperature and bias current .
Most important, the polarization dependent single mode behavior as a function of bias and RF power.
Dynamic measurements: S21, S11. Combined AM and FM responses at 3.4 [GHz] and extraction of the linewidth enhancement factor .
Long term measurements: stabilizing the temperature and supplying a static current, we measured the wavelength for more than 15 days, and the light power for over one year.
The lasers made at Technion were found to be superior to those made by VIXAR. This manifests itself in several key parameters. The most important one is the temperature dependent threshold. The threshold of the Technion made lasers is smaller at 900 [C] compared to room temperature. This, rather unusual property is due to the fact that the lasers were specifically designed for the high temperature. A second crucial characteristic is the fact that the lasers exhibit single mode behavior and are polarized well at bias levels up to 1.7-1.8 [mA] well above the VIXAR specification. Finally, the modulation response is wider and the output power is higher.
* M.Sc. student under the supervision of Prof. Gadi Eisenstein.