We have demonstrated a compact and efficient frequency stabilization system based on Pound-Drever-Hall method,
along with optical feedback. The frequency of a 30mW 405nm GaN violet laser diode (LD) was stabilized to a reference
confocal Fabry-Perot cavity (CFP cavity) by negative electrical feedback to the injection current of the LD based on
Pound-Drever-Hall technique. Moreover, by employing optical feedback from another tilted CFP cavity, the residual
frequency noise has been efficiently suppressed. The minimum square root of the Allan variance was 1.65×10-11 at the
integration time of 0.5s under the optical-electrical double feedback condition. We have achieved the stabilization of
visible violet LD by optical feedback method for the first time.
We have demonstrated a compact and inexpensive frequency stabilization technique for commercially available 1mW,
850nm vertical cavity surface emitting laser (VCSEL) using a Fabry-Perrot cavity as frequency standard. Recently
VCSEL has been widely prevailed for uses of low cost and small sized sensors, since it may afford low power
operations and manufacturing costs in comparison with edge emitting type Fabry-Perrot laser diodes. Therefore, a
highly versatile and inexpensive frequency stabilized coherent light source which can be mass producible will be
available if the frequency stabilization for this type of VCSEL's is carried out. Generally, it has been commonly
accepted that a satisfactory degree of coherence may be easily obtained from VCSEL's without any additional frequency
stabilization technique since highly reflective coatings are to be put on their laser cavity edges. Nevertheless, some
VCSEL devices, especially inexpensive type commercial products show multi-mode behaviors along with polarization
instabilities. In the present work, as a simple and inexpensive approach to commercially available VCSEL devices, we
have demonstrated a frequency stabilization scheme using a Fabry-Perrot cavity. The error signal was derived by phase
sensitive detection for the transmitted light from the Fabry-Perrot resonator. Thus, the lasing frequency of the VCSEL
was locked to the zero-crossing of the error signal by negative feedback for injection current via a PID controller. As a
result, we have successfully suppressed the amount of frequency fluctuations in the free-running VCSEL of as much as
2GHz to be within 500MHz at measuring time of 30sec, that is, the attained Allan variance is 4.1×10E-8.
KEYWORDS: Semiconductor lasers, Laser stabilization, Signal detection, Light sources, Frequency modulation, Fermium, Digital holography, Computing systems, Control systems, Geometrical optics
In the present work, we have developed an efficient and well stablized hyper coherent diode laser light source as
compact as even portable using commercially available visible 400 nm band laser diodes. The attained coherence of the
present system can always be controlled at the best condition indifferent to changes in its settled environmental
conditions by applying Pound-Drever-Hall technique in which the frequency of a 160mW type 405nm GaN violet laser
diode is locked to a reference Fabry-Perot cavity by negative electrical feedback for the injection current of the laser
diode based on FM sideband technique. In addition to this frequency stabilization system, we have also realized a
stability evaluation system that can measure the Allan variance of the frequency fluctuations of our frequency stabilized
laser source in real-time basis by using simple devices of a portable computer and a digital signal processing unit. As a
result, we have accomplished a compact and efficient hyper coherent laser system which can always perform its
optimum conditions even if the environmental conditions around the laser are to be dynamically changed when used in a
field basis. The attained values of power spectral density (PSD) of FM noise calculated from the error signals of our
system under controlled condition were better by about 1~2 orders than typical values of free-running conditions in the
fourier frequency domain from 100Hz to 300kHz. The best achieved value of PSD was about 2.56×107 [Hz2/Hz] in the
fourier frequency domain from 100Hz to 1kHz, while as for the Allan variance as another measure of frequency
stability, the achieved value of the minimum square root of Allan variance was 3.46×10-11 in a 400nm type violet laser
diode at integration time of 10 ms, which has been well comparable to the hyper coherent condition for the laser diode
light sources.
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