FM NOISE-REDUCTION AND SUBKILOHERTZ LINEWIDTH OF AN ALGAAS LASER BY NEGATIVE ELECTRICAL FEEDBACK

Negative electrical feedback was applied to a CSP-type AlGaAs laser, by which its FM noise was reduced at the Fourier frequency range of f ≦ 40 MHz. The magnitude of the FM noise was far lower than the quantum noise level of the free-running laser at 100 Hz ≦ f ≦ 4.4 MHz. Especially, it was as low as 1 × 10− − 1 × 10−6 that of the free-running laser at 100 Hz ≦ f ≦ 1 kHz. The full width at half maximum of the field spectrum was reduced to 560 Hz. The major factors necessary for realizing the very low FM noise level were 1) the presently used laser had almost constant FM response characteristics for a wide Fourier frequency range, 2) a high-finesse Fabry-Perot interferometer was employed for highly-sensitive FM noise detection and to get higher feedback gain, 3) the reflection mode of the Fabry-Perot interferometer was employed to increase the bandwidth and efficiency of the FM noise detection, and 4) a computer simulation was utilized for optimum design of the feedback loop. © 1990 IEEE