An inordinate amount of time, effort (and paper) has been spent trying to find a way to stabilize laser-diode frequencies, but our research team has been working on the premise, that frequency instability can, in fact, have its up-sides. In the present work, we focus on a method that uses laser diodes’ own noise to generate physical random numbers. Introducing a frequency discriminator as a reference, we control and stabilize the difference between the frequency reference and the laser frequency, thereby generating random numbers at a suitable point.
An optical range finder system that relies on laser diodes’ frequency noise, instead of intensity or frequency modulations, and its improvement in resolution are reported. The distance to the target is measured by calculating the cross-correlation of two signals reflected from the target and reference mirrors. These two signals are converted from the laser diodes’ frequency noise signals by frequency/intensity converters, such as a Fabry–Perot etalon. We obtained the distance to the target by checking time lags between the target and reference beams at the highest correlation coefficient. We also measured the change in the correlation coefficient around the peak sampling point by adjusting the reference-path length, achieving a resolving power of ±3 mm.
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.
Random numbers can be classified as either pseudo- or physical-random, in character. Pseudo-random
numbers are generated by definite periodicity, so, their usefulness in cryptographic applications is
somewhat limited. On the other hand, naturally-generated physical-random numbers have no calculable
periodicity, thereby making them ideal for the task. Diode lasers' considerable wideband noise gives them
tremendous capacity for generating physical-random numbers, at a high rate of speed. We measured a
diode laser's output with a fast photo detector, and evaluated the binary-numbers from the diode laser's
frequency noise characteristics. We then identified and evaluated the binary-number-line's statistical
properties. We also investigate the possibility that much faster physical-random number
parallel-generation is possible, using separate outputs of different optical-path length and character, which
we refer to as "coherence collapse".
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.
Even as long ago as the 1960's, scientists understood that diode lasers' oscillation wavelengths demonstrated significant
shifts to the shorter wavelength side, when subjected to strong magnetic fields, at extremely low temperatures. When we
exposed Fabry-Perot type diode lasers oscillating at 780 nm to weak magnetic fields, at room temperature, the oscillation
wavelength was observed to have shifted to the longer wavelength side. In discussions of shift mechanisms aimed at
explaining how/why our results differ from those obtained in studies conducted in the 1960's, we noted a rise in
temperature and an increase in the carrier density, and how it affected the characteristic shifts observed, when a magnetic
field was applied to the Fabry-Perot type laser diodes parallel to the injection current. In the present work, we tested the
oscillation wavelength shift of a vertical-cavity surface-emitting laser (VCSEL) in a magnetic field, because we expected
that, by doing so, the VCSEL would show a shorter wavelength side shift.
Random numbers can be classified as either pseudo- or physical-random in character. Pseudo-random numbers'
periodicity renders them inappropriate for use in cryptographic applications, but naturally-generated physical-random
numbers have no calculable periodicity, thereby making them ideally-suited to the task. The laser diode naturally
produces a wideband "noise" signal that is believed to have tremendous capacity and great promise, for the rapid
generation of physical-random numbers for use in cryptographic applications. We measured a laser diode's output, at a
fast photo detector and generated physical-random numbers from frequency noises. We then identified and evaluated the
binary-number-line's statistical properties. The result shows that physical-random number generation, at speeds as high
as 40Gbps, is obtainable, using the laser diode's frequency noise characteristic.
We introduced the vertical cavity surface emitting laser (VCSEL) as the laser diode in tour external cavity system.
Because VCSELs are now commercially available, and the External cavity diode laser (ECDL) systems using
them are expected to improve their frequency stability, we have replaced a Fabry-Perot type laser diode with a
VCSEL, and examined its oscillation-frequency stability. Therefore we were able to expect that the VCSELs with
our double optical feedback system have good oscillation frequency stability. The obtained VCSEL's
oscillation-frequency stability, i.e., the square root of Allan variance σ was 4×10-10, at an averaging time of τ=1
sec.
Random numbers can be classified as either pseudo- or physical-random in character. This work demonstrates how
laser diodes' inherent noise can be exploited for use in generating physical-random numbers in cryptographic
applications. In the initial stages of the experiment, we measured a laser diode's output, at a fast photo detector and
generated physical-random numbers from intensity noises. We then identified and evaluated the binary-number-line's
statistical properties. Our preliminary results show that fast physical-random numbers are obtainable, using the laser
diode's frequency noise characteristics.
Ongoing tests involving the application of double optical feedback to a vertical-cavity surface-emitting laser (VCSEL) are resulting in demonstrable and significant improvements in oscillation linewidth and frequency stability. We look into the workings of a double optical feedback system for a Fabry-Pérot-type diode laser. We describe a single-mode VCSEL that is characterized by a narrow oscillation linewidth and a stable output intensity that shows no trace of the low-frequency fluctuation (LFF) that so often occurs in single optical feedback diode lasers. Initial tests use a beat note to do this. From there, we calculate the square root of the Allan variance to determine the level of frequency stability. We also evaluate the degree of LFF suppression achieved using its spectrum density and compare those results with what we obtained through single and double optical feedback and with no feedback whatsoever.
The precise interferometric systems employed in today's artificial satellites require semiconductor lasers of the
highest caliber. To this end, efforts to stabilize their oscillation frequencies and narrow spectrum line-widths
continue relentlessly. While a number of different approaches have been tested, none have provided overall,
long-term stability. Most recently, we employed a Doppler-free absorption line of Rb atoms, with a precision
temperature controller and an improved laser mount. In this instance, relative optical frequency stability rated
9.07×10-13≤σ(2,τ)≤7.54×10-10, in averaging time for 0.01s≤τ23s. By introducing an optical feedback, which
narrows the laser's linewidth, we obtained improved frequency stability.
External cavity diode lasers (ECDL) are presently experiencing a surge in popularity, as laser light-sources for advanced
optical measurement systems. While these devices normally require external optical-output controls, we simplified the
setup, a bit, by adding a second external cavity. This technique boasts the added advantage of having a narrower
oscillation-linewidth than would be achievable, using a single optical feedback. Because drive-current and atmospheric
temperature directly impact the ECDL systems' oscillation frequency, during frequency stability checks, it was
necessary, in this instance, to construct a slightly smaller ECDL system, which we mounted on a Super-Invar board, to
minimize the influence of thermal expansion. Taking these and other aggressive and timely measures to prevent
atmospheric temperature-related changes allowed us to achieve an improvement in oscillation-frequency stability, i.e.,
to obtain the square root of Allan variance σ =2×10-10, at averaging time τ =10-1.
We introduced a vertical-cavity surface-emitting laser (VCSEL) to the setup, for the simple reason that its frequency is
far less susceptible to changes in temperature, than other lasers of its type. And, because VCSELs are widely available,
and the ECDL systems that use them improve frequency stability, we replaced the Fabry-Perot semiconductor laser with
a VCSEL.
The precise interferometric systems employed in today's artificial satellites require semiconductor lasers of the highest callibur. But,
one particularly large obstacle has stood in the way of their broad application; the stabilization of their oscillation frequencies. While a number of different approaches have been tested, none have provided overall, long-term stability. Most recently, we used a Doppler-free absorption line of Rb atoms with a precision temperature controller and an improved laser mount; in this instance, relative optical frequency stability rated 9.07x10-13≤&sgr;(2,&tgr;)≤7.54x10-10, in averaging time for 0.01s≤&tgr;≤23s. By introducing optical feedback, which narrows the laser's linewidth, we obtained improved frequency stability.
External cavity diode laser (ECDL) systems are presently experiencing a surge in popularity as laser light-sources, in
advanced optical communications- and measurement-systems. Because such systems require that their external
reflectors be precisely controlled, to eliminate low frequency fluctuations (LFF) in optical output, we conducted
experiments with a two-cavity version, which easily eliminated LFFs, as expected. The technique has the added
advantage of a narrower oscillation-linewidth than would be achievable, using a single optical feedback. However, the
ECDL's oscillation frequency is susceptible to the influences of the drive-current, as well as changes, both in the
refractive index, and the overall length of the external reflector that results from fluctuations in atmospheric temperature.
We made every effort to maintain the length of the ECDL cavity, while evaluating oscillation-frequency stability. We
used a Super-Invar board as the platform for our compact ECDL system to minimize the influence of thermal expansion,
because of its low expansion coefficient. We then compared the effect of atmospheric temperature variations between
two experimental conditions, with the Super-invar board and without it, and finally took note of the improvement in
performance, using the board.
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 (LFF) in optical output, we conducted experiments with a two-cavity version of ECDL system, which was expected to eliminate LFF easily. This technique brings the added advantages of a narrower linewidth than would be achievable via a single optical feedback. However, the ECDL's oscillation frequency is susceptible to the influences of the driving current, changes in the refractive index, and the expansion/contraction of the length of the external reflector that results from fluctuations in atmospheric temperature. We made every effort to maintain the length of the ECDL cavity, while evaluating oscillation-frequency stability. We used a super-inver board as the platform for our ECDL system, in order to minimize the influence of thermal expansion. Moreover, our ECDL system combines an Rb cell within an external cavity to improve stability; by restricting the LD frequency to both the external cavity mode and to the Rb saturated absorption spectrum. We used the square root of the Allan variance to evaluate oscillation frequency stability, observing, in the process, that it improved stability about one order of magnitude.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.