While optical laser range finders use random signals to determine distance, a laser diode’s fast frequency noise can perform the task. Moreover, this signal can be applied to physical-random number generation. This research describes a method, whereby laser diode’s frequency noise characteristics generate a large number of physical-random numbers and determine the distance to a target [1] [2]. We tested the random number generating- and distance- measuring capabilities of two types of lasers; a Fabry-Perot-LD and VCSEL: (Vertical Cavity Surface Emitting Laser). With the Fabry-Perot etalon functioning as frequency discriminator, we investigated the physical-random numbers’ characteristics from both Fabry-Perot-LD’s and the VCSEL’s characteristic’s points of view. We verified the generated binary number’s randomness, using NIST FIPS140-2 test, and noted the Random Number Generation (RNG) speed of a FP-LD was 48 Gbit/s, and that of a VCSEL was 159 Gbit/s. When the generation speed of the physical-random number is high, we can increase the sampling rate of our range finders and improve resolution.
Not so long ago, pseudo random numbers generated by numerical formulae were considered to be adequate for encrypting important data-files, because of the time needed to decode them. With today’s ultra high-speed processors, however, this is no longer true. So, in order to thwart ever-more advanced attempts to breach our system’s protections, cryptologists have devised a method that is considered to be virtually impossible to decode, and uses what is a limitless number of physical random numbers. This research describes a method, whereby laser diode’s frequency noise generate a large quantities of physical random numbers. Using two types of photo detectors (APD and PIN-PD), we tested the abilities of two types of lasers (FP-LD and VCSEL) to generate random numbers. In all instances, an etalon served as frequency discriminator, the examination pass rates were determined using NIST FIPS140-2 test at each bit, and the Random Number Generation (RNG) speed was noted.
Semiconductor laser range-finder systems use so-called “time-of-flight” methods that require us to modulate semiconductor lasers’ intensity and frequency, and detect those of reflected lights, in order to compare optical paths to the reference and the target. But, accurate measurement requires both high-speed modulation and detection systems. By taking advantage of semiconductor lasers’ broad- spectrum frequency noise, which has a range of up to a few GHz, and converting it to intensity noise, we were able to generate a set of high-speed physical random numbers that we used to precisely measure the distance. We tuned the semiconductor lasers’ oscillation frequencies loosely to the Rb absorption line and converted their frequency noise to intensity noise, in the light transmitted. Observed through a frequency discriminator, beams traveling along two different paths will always share intensity noise patterns, but there is a time lag. We calculate the cross-correlation of the two signals by sweeping their time lags. The one with the highest degree of correlation was that corresponding to the difference in the length of the two optical paths. Through our experiments, we confirmed that the system was accurate up to a distance of 50 m, at a resolution of 0.03 m, when the sampling rate was adjusted to 0.2 ns.
We describe a method of generating physical random numbers by means of a diode laser that has an extremely wide-band frequency-noise profile. Fluctuations in the laser frequency affect the intensity of the light transmitted through the optical frequency discriminator, detected thereafter as random fluctuations. This allows us to simultaneously generate 8 random bit streams, due to the parallel processing of 8-digit binary numbers sampled by an 8-bit analog-to-digital converter. Finally, we generated physical random numbers at a rate of 3 Gbit/s, by combining one data stream with another stream that is delayed by 2 ms, by exclusive-OR.
External cavity diode laser (ECDL) systems are presently experiencing a surge in popularity as laser
light-sources, in advanced optical communications- and measurement-applications. Because such
systems require that their external reflectors be precisely controlled, to eliminate low frequency
fluctuations in optical output, we conducted experiments with a two-cavity version of the ECDL
system for a vertical cavity surface emitting laser (VCSEL). This technique brings the added
advantages of a narrower linewidth than would be achievable via a single optical feedback.
VCSELs are characterized by wider oscillation linewidths than edge emitting types, so the larger
effect of double optical feedback system is expected.
As long ago as the 1960s, scientists understood that Diode lasers' oscillation wavelengths showed a
significant shift to the shorter wavelength (high frequency) side, when exposed to strong (<4[T])
magnetic fields, at extremely low temperatures (<80[K]). Not surprisingly, then, in preliminary tests,
when we exposed Fabry/Perot-type diode lasers oscillating at 780[nm] to weak magnetic fields (<1.4[T]),
at room temperature (300[K]), we observed that the oscillation wavelength shifted to the longer (low
frequency) wavelength side. In the present work, we used vertical-cavity surface-emitting lasers (VCSEL)
to check whether its change into the shorter wavelength side takes place.
In discussions of shift mechanisms, we consider how wavelength (frequency) and optical output-power
shifts are correlated. Our expanded knowledge base has forced us to use a completely different
mechanism to explain how/why our results differ from those obtained in studies conducted in the 1960's.
We are now introducing a mechanism that affects a rise in temperature and an increase in the carrier
density, affect the characteristic shifts observed in our experiments, when a magnetic field is applied to
the laser diodes parallel to the injection current.
We have succeeded, thus far, in stabilizing laser diode (LD) frequencies to Rb absorption lines, by means of negative electrical
feedback. While the absorption lines were stable over the long term, the Doppler Effect's influence was evident, in broadened spectrum
linewidth. To avoid the problem in subsequent tests, we used Rb-saturated absorption signals.
In this work, we demonstrated potentials of two applications; one was as a reference signal source for estimation of other LD's
frequency stabilization system, and the other was a light source for generation of THz radiation.¬«W
The semiconductor lasers in use today are on one hand, prized, and highly praised, for their small size, light weight, longevity and
energy-efficiency, -and on the other, criticized for their susceptibility to frequency-fluctuations brought about by changes in
temperature and driving current. Once this "wrinkle" is ironed out, semiconductor lasers will become the default light-sources, for
satellites' onboard interferometers. Our studies have been directed at stabilizing oscillation frequency to the atomic absorption line,
and using negative electrical feedback to the injection current. Frequency stabilization is accomplished, by either; a) applying direct
modulation to the semiconductor laser's driving current, or b) modulating the reference frequency, to obtain the error signal needed
for stabilization. In this instance, Faraday effect-based stabilization was used. This indirect oscillation frequency stabilization has no
discernable effect on spectra width, but, stability was no better than that observed in the system using the direct modulation.
When we compared Faraday effect- and direct modulation-based methods of stabilization, in order to uncover the root-cause of the
discrepancy, sensors picked up system noise, the source of which was heat generated by the heavy current applied to a magnetic coil
used to apply the Faraday effect. We also substituted a permanent magnet for the electromagnet.
Scientists throughout the world are seeking to enhance the capabilities of satellite-to-satellite tracking laser
interferometer-based optical systems used to measure the alterations in earth's gravitational field that indicate critical
changes in the environment. These systems must be able to measure infinitesimal fluctuations in the relative velocities of
two satellites, using a light source that oscillates at a level of frequency stability rated better than 10-13 in the square root
of the Allan variance. In our experiments, semiconductor laser frequency stabilization that typically requires a brief
direct modulation of the laser injection current to obtain an error signal, was accomplished using the Faraday effect of Rb
absorption lines. This effectively modulates the reference frequency of the stabilization system, i.e., the Rb absorption
line, by modulating the magnetic field applied to the Rb absorption cell, instead of the oscillation frequency of the laser
diode. Most recently, we used the Faraday method, in conjunction with a precision temperature controller. For present
purposes, we also use the PEAK method, to obtain the most accurate signal possible, comparing it with saturated
absorption spectroscopic readings, to determine the noise-source.
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