Modulation Properties of Injection Lasers.
Abstract
A high resolution optical measurement setup consisting of a monochromator and a Fabry-Perot interferometer was developed with a resolution capability of 30 MHz in the 0.8 to 0.9 (mu)m wavelength range. The measurement setup was used to display the spectra of modulated and unmodulated GaAs heterostructure injection lasers. The unmodulated laser longitudinal and transverse mode behavior was carefully analyzed. It was concluded that no laser transverse modes exist, which is in contrast to earlier reports. All tested lasers from three different manufacturers showed a new spectra detail called "Fine -structure Ripple" (FR). The FR is a periodic ripple that appears on both sides of each longitudinal laser mode, at a frequency spacing of typically 1 to 2.5 GHz. The FR is bias current dependent, and becomes extremely pronounced when lasers are operated in their saturated regions. The pronounced FR of such a hard driven laser is caused by bias current self-modulation. A theoretical treatment based on an FP-cavity model for the laser will be presented. A laser line-shape function was derived that predicts the FR that is well in agreement with our experiments. Decomposition of the laser's FP electromagnetic field into a finite number of terms, and gain saturation effects, are key elements used in this development. Modulation experiments of injection lasers at microwave frequency rates demonstrate that bias current modulation results primarily in a frequency modulated laser output signal. Amplitude modulation is a secondary effect. This experimental finding is in close agreement with the theoretical predictions based on the above FP -cavity model. FM modulation indices of 1.45 and 2.4 at frequencies of 4.0 and 3.3 GHz respectively, were achieved. Separate AM and FM laser communications systems tests are presented. For this purpose, an optical AM detector, in the form of a GaAs MESFET, was developed and successfully tested to frequencies beyond 10 GHz. The static frequency sensitivity of a laser oscillator was analyzed and calculated. The results compare well with measured sensitivities of 4 to 8 GHz/mA. A theoretical treatment of the laser as a parametric device is presented. The theory has general applications beyond the scope of this thesis. The case of a nonlinear reactive upconverter is selected and used to calculate the laser's upconversion gain. For a 1 GHz microwave to IR signal conversion, a gain of 28 db is predicted; well in agreement with our measurements. The connections between the new FP-cavity modulation theory, the parametric laser modulation theory, and the existing modulation theory based on the rate equations, are made. Limitations and features of the three different approaches are discussed. A more sophisticated modulation theory is presented in the appendix which is based on Lamb's optical laser model. Frequency modulation is demonstrated by using the dependence of the oscillation frequency on the polarization field which can be modulated by the laser bias current. The modulated laser field and spectrum are calculated. The theory makes use of a quantized two-level atomic model to describe the pumped laser medium, but uses a classical laser radiation field.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- March 1980
- Bibcode:
- 1980PhDT........17O
- Keywords:
-
- Physics: Optics;
- Frequency Modulation;
- Gallium Arsenide Lasers;
- Injection Lasers;
- Optical Measurement;
- Electromagnetic Fields;
- Fabry-Perot Interferometers;
- Field Effect Transistors;
- Laser Outputs;
- Lasers and Masers