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".
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.
The primary objective of the Decihertz Interferometer Gravitational Wave Observatory (DECIGO) mission is to detect and observe black holes' and galactic binaries' gravitational waves (GWs), at frequencies ranging from 10−2 to 101 Hz (from 0.1 to 100 s in averaging time). This low-frequency range is inaccessible to ground-based interferometers, due to unshieldable background noise and to the fact that ground-based interferometers are limited to a few kilometers in length. Our research is focused on efforts to stabilize semiconductor, Nd:YAG, and fiber lasers, for use as GW detectors' optical sources. In present-day and future detectors, frequency and phase noise may place certain limitations on sensitivity and stability. Our goal is primarily to design robust experiments. In this report, we compare existing methods: Faraday, Faraday peak, and saturated absorption spectroscopy. In these, the laser frequency is stabilized to Rb as an atomic frequency reference by a feedback-loop control system. From the frequency stability of these models, we can predict the characteristics of the three systems through dynamic stability analysis, by analyzing the dynamic Allan variance. We find the optical frequency stability, expressed as the Allan deviation (the square root of the Allan variance), to be 3.3×10−11, 2.9×10−12, and 1.2×10−12 in the respective methods.
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.
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.
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 9.07×10-13 ≤ σ(2,τ) ≤ 7.54×10-10, in averaging time for 0.01s ≤ τ ≤ 23s. Furthermore, we heated the Rb cell to up to 313K, in order to enhance the control signal and improve oscillation frequency stability.
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.
Laser interferometers detect gravitational waves with a degree of accuracy and efficiency unimaginable even a few years
ago. The semiconductor lasers that are the primary light source for these devices are small, lightweight, durable and
energy-efficient. On the downside, the devices currently available are still marked by broad oscillation spectra, and
heightened sensitivity to fluctuations in injection current and /or ambient temperature. By applying a small sine wave to
the injection current, we modulate the oscillation frequency. This frequency-modulated beam is introduced to the
Avalanche photo diode through the Rb cell in the saturated absorption optical setup. The resulting signal and a reference
signal are detected simultaneously and combined, to produce an error signal, which, when fed back to the injection current,
stabilizes the diode's oscillation frequency at 2.12x10-12 ⩽ &sgr;(2,τ) ⩽ 5.88x10-11 in the averaging time between 0.4s to 65s.
An optical feedback method, which introduces the laser beam reflected by a mirror or a grating to the semiconductor laser
itself, is reported to narrow oscillation linewidth and improve frequency stability. We are now combining these two
techniques to further improve frequency stability.
A method of detecting gravitational-field variations using laser diodes is described. While the GRACE project is
currently using the Doppler microwave system to measure the velocities of satellites flying in tandem, in the future, more
advanced laser interferometry will be employed. It is hoped that we will be able to measure infinitesimal changes in their
velocities, by using frequency-stabilized lasers rated at better than 10-13 in the square root of the Allan variance (&sgr;) for 1s
< τ < 100s. As laser light sources, these devices will be notable for their compactness, energy efficiency, lightweight and
high frequency-stability. This thesis describes the improved frequency stabilization obtained through the use of the
magneto-optical effect of the Rb-D2 absorption line, and the adaptation of the PEAK method, in order to obtain a precise
control signal. The method allows us to modulate the reference frequency of the stabilization system (the absorption
spectrum of the Rb-D2 absorption line) by modulating the magnetic field applied to the Rb absorption cell, instead of the
oscillation frequency of the laser diode. In so doing we are able to achieve a frequency stabilized laser diode (&sgr; = 9 x
10-12), while maintaining its linewidth, at an averaging time of 40s. In the next stage, we will test frequency-stabilized
laser optical sources that are to be used in detecting and observing gravitational waves.
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.
The present work discusses methods of stabilizing the frequencies of commercially-available laser diodes. Laser diodes are generally compact and long-lived. The frequency stability, which makes them ideal for onboard laser interferometer light-sources, in applications such as the satellite-to-satellite tracking systems used to verify fluctuations in earth's gravity field, which, in turn, indicate other critical changes in the environment, is the key characteristic of this work. We used the devices typically operating at 780nm, and their frequencies can be stabilized using either of two systems; one, employing the Doppler-free absorption line of Rb atoms and another, sing the Faraday effect of the Rb absorption line. In both cases, the use of the proper modulation frequency and amplitude improved frequency stability, overall, attaining 2.05×10-12 and 2.73×10-11, respectively, in the square root of the Allan variance, by measuring the beat-note between two independently-stabilized laser diodes.
We report on the stabilization of a semiconductor laser's frequency, using Rb absorption lines. In order to improve overall frequency stability within our system, we adjusted the setup used in Rb- saturated absorption spectroscopy, and optimized modulation parameters such as modulation - frequency and -width, to more accurately detect the error signal. When we stabilized laser frequency using a Doppler-free absorption line of Rb atoms, a time-constant of 0.01sec, and a modulation frequency of 7.77kHz, relative optical frequency stability of 2.12×10-12≤σ(2,τ)≤5.88×10-11 was achieved, in averaging time for 0.04s≤τ≤65s.
We report on the stabilization of a semiconductor laser’s frequency, using spectra-controlled etalon. As the spectra of an etalon are controlled by one of the Rb absorption lines, they provide highly stable reference frequencies in a broad frequency range. When we adapted the PEAK method to the etalon’s spectra and used a Doppler-free absorption line of Rb atoms as the control signal for the newest model of our system, relative optical frequency stability of 2.91x10-11≤σ(2,τ)≤3.72x10-10 was achieved in averaging time for 0.04s≤τ≤100s.
Currently in the initial stages of development, the endeavor aims to use satellite-to-satellite tracking laser interferometer-based optical technique, to document fluctuations in earth’s gravitational field indicating other critical changes in the environment. This system must be able to measure infinitesimal changes in the relative velocity of the two satellites, using a laser light source, which oscillates at frequency stability better than 10-13 in the square root of the Allan variance. We have stabilized the laser’s oscillation frequency using the Faraday effect of Rb absorption lines. This method modulates the reference frequency of the stabilization system by modulating the magnetic field applied to the Rb absorption cell, instead of the oscillation frequency of the laser diode. Furthermore, we have adapted the “double optical feedback” to the laser diode for narrowing its oscillation spectrum and improving its frequency stability. In recent years, a “femtosecond optical comb generator” has been developed as a new reference frequency source for absolute frequency measurement. This optical comb generator is controlled by the microwave frequency standards systems and provides stability of 4x10-13 at an averaging time of 1s and at the order of 10-15 at 1000s averaging time. We have measured the frequency stability of our system using the optical comb. We obtained the best spectrum narrowing effect using two gratings as external reflectors in the double optical feedback setup. The obtained results were 6.269x10-11 ≤ σ ≤ 1.516x10-10 (24.11kHz ≤ f ≤ 58.31kHz) from 1s to 39s in the averaging time.
Currently in the initial stages of development, the endeavor aims to use satellite-to-satellite tracking laser interferometer-based optical technique, to document fluctuations in earth's gravitational field indicating other critical changes in the environment. This system must be able to measure infinitesimal changes in the relative velocity of the two satellites, using a laser light source, which oscillates at frequency stability better than 10-13 in the square root of the Allan variance. The constraints placed upon the system will ultimately require that the interferometer's light-source be small, lightweight, extremely efficient, durable, and possess high frequency-stability. Present-day technology of a laser diode possesses all of these characteristics, except the last and most critical one; frequency stability. So, our ongoing efforts are all focused on the search for a method of stabilizing the oscillation frequency of the satellite-to-satellite laser-diode light source. We used a Rubidium absorption line as a frequency reference of the frequency stabilization system and the "double optical feedback" method to narrow the laser diode's oscillation linewidth, in order to improve its frequency stability. We have measured the frequency, to determine its degree of stability, by comparing the "femtosecond optical comb generator"- and laser diode-frequencies.
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