Long-term Absolute Wavelength Stability of Acetylene-stabilized Reference Laser at 1533 nm , Journal of Telecommunications and Information Technology, 2016, nr 4

The second harmonic generation process in Periodically Poled Lithium Niobate (PPLN) has been applied in order to measure frequency of reference laser locked to acetylene absorption peak 12 12 12C2 2 2H2 2 2 (P13) (1533 nm) against optical frequency synthesizer. The measurement results have been compared to the results obtained using different techniques for the same reference laser during the past 10 years in other laboratories

SEMICONDUCTOR FREQUENCY STANDARD BASED ON P(16) SPECTRAL LINE OF ACETYLENE ISOTOPE WITH TEMPERATURE STABILIZATION BY PHASE MODULATION

This paper reviews the method of semiconductor laser diode frequency stabilization by phase modulation. Also parameters are identified that affect the quality of stabilization and the estimation of Allan deviation is obtained. The pilot setup has been put together and it consists of: a semiconductor distributed feedback laser diode, a fiber phase modulator, an electrical signal generator, an acetylene-13 isotope cuvette, a photodetector, a lock-in amplifier and personal computer for measurement processing. Modulated laser diode radiation passed through a gas cell provides information about the position of radiation spectral line relative to the center of gas spectral line. Gas molecular spectral lines provide frequency standard with low sensitivity to external effects. When using the reference signal, one can get an error signal in a lock-in amplifier that changes the laser diode temperature and, as a result, its wavelength. Allan deviation was estimated based on measured frequency data. Long-term stability can be improved in the time range between 0.1 s and 100 s up to 1∙10-8 (Allan deviation). This method of stabilization is useful for the development of compact high reliable optical frequency standards for space applications

Advanced applications of wavelength tunable lasers in metrology and fundamental physics

In this thesis, measurement systems based on wavelength tunable lasers have been developed and studied to improve measurement capabilities in the fields of optical metrology and quantum optics. In the first work presented in this thesis, techniques for precision measurements of absolute spectral irradiance responsivity of detectors were investigated. Two laser-based methods and a traditional monochromator based method were compared in the near infrared wavelength region. The results between absolute responsivity measurements using the three different measurement systems demonstrated agreement at the uncertainty level of less than 0.1 % (k = 1). The second work consists of an acetylene-stabilized laser and an optical single-frequency synthesizer that were constructed and characterized for precision optical frequency measurements at telecommunication wavelengths. The acetylene-stabilized laser was designed by taking a fiber-based approach, which enabled a relatively straightforward implementation of the optical set-up. The frequency of the stabilized-laser was measured absolutely using an optical frequency comb generator. The results agree well with the recommendation by the International Committee for Weights and Measures (CIPM). The single-frequency synthesizer was designed for generating a single user-specified frequency from an atomic time base within the 192-196 THz gain bandwidth of an erbium-doped fiber amplifier (EDFA). The synthesizer was utilized for studying spectral lineshapes of acetylene transitions near 1540 nm. The recorded spectra were investigated by theoretical fits and the obtained line-center frequencies were compared to line-center frequencies measured with the acetylene-stabilized laser using the third harmonic technique. The results agreed well with each other. Final part of the thesis describes a set-up that is capable of emitting indistinguishable single photons using single molecules as photon sources. This was achieved by combining high resolution laser spectroscopy and optical microscopy at cryogenic conditions. Two single molecules in separate microscopes were identified and DC-Stark effect was exploited to shift the resonance frequencies of given molecules for perfect spectral overlap. Excitation by pulsed laser enabled triggered generation of identical single photons from two independent single molecules. The results can be utilized in the development of a number of different quantum information processing schemes

Optical frequency references in acetylene-filled hollow-core optical fiber and photonic microcells

Doctor of PhilosophyDepartment of PhysicsKristan L. CorwinOptical frequency references have been widely used in applications such as navigation, remote sensing, and telecommunication industry. For stable frequency references in the near-infrared (NIR), lasers can be locked to narrow absorption features in gases such as acetylene. Currently, most Near NIR references are realized in free space setups. In this thesis, a low-loss hollow-core optical fiber with a diameter of sub millimeters is integrated into the reference setup to provide long interaction lengths between the filling gas and the laser field, also facilitate the optical interaction with low power levels. To make portable NIR reference, gas can be sealed inside the hollow-core fiber, by creating a photonic microcell. This work has demonstrated all-fiber optical frequency references in the Near IR by fabricating and integrating gas sealed photonic microcells in the reference setup. Also, a thoughtful study regarding the lineshape of the fiber-based reference has been accomplished. According the proper modeling of a shift due to lineshape, a correction was applied to our previous absolute frequency measurement of an NIR optical frequency reference. Furthermore, effects of the hollow-core fibers, including mode-dependence frequency shift related to surface modes are explored. In addition, angle splicing techniques, which will improve the performance of the fiber-based frequency reference have been created. Low transmission and return loss angle splices of photonic bandgap fiber, single mode PCF, and large core kagome to SMF-28 are developed and those fibers are demonstrated to be promising for photonic microcell based optical frequency references. Finally, a potentially portable optical metrology system is demonstrated by stabilizing a fiber-laser based frequency comb to an acetylene-filled optical fiber frequency reference. Further work is necessary to fabricate an all-fiber portable optical metrology system with high optical transmission and low molecular contamination

Ultra-stable microwave generation with a diode-pumped solid-state laser in the 1.5-µm range

We demonstrate the first ultra-stable microwave generation based on a 1.5-µm diode-pumped solid-state laser (DPSSL) frequency comb. Our system relies on optical-to-microwave frequency division from a planar-waveguide external cavity laser referenced to an ultra-stable Fabry–Perot cavity. The evaluation of the microwave signal at ~10 GHz uses the transportable ultra-low-instability signal source ULISS®, which employs a cryo-cooled sapphire oscillator. With the DPSSL comb, we measured −125 dBc/Hz phase noise at 1 kHz offset frequency, likely limited by the photo-detection shot-noise or by the noise floor of the reference cryo-cooled sapphire oscillator. For comparison, we also generated low-noise microwave using a commercial Er:fiber comb stabilized in similar conditions and observed &gt20 dB lower phase noise in the microwave generated from the DPSSL comb. Our results confirm the high potential of the DPSSL technology for low-noise comb applications

Frequency Stabilization of DFB Laser Diodes at 1572 nm for Spaceborne Lidar Measurements of CO2

We report a fiber-based, pulsed laser seeder system that rapidly switches among 6 wavelengths across atmospheric carbon dioxide (CO2) absorption line near 1572.3 nm for measurements of global CO2 mixing ratios to 1-ppmv precision. One master DFB laser diode has been frequency-locked to the CO2 line center using a frequency modulation technique, suppressing its peak-to-peak frequency drifts to 0.3 MHz at 0.8 sec averaging time over 72 hours. Four online DFB laser diodes have been offset-locked to the master laser using phase locked loops, with virtually the same sub-MHz absolute accuracy. The 6 lasers were externally modulated and then combined to produce the measurement pulse train

Applications of Mid-infrared Frequency Combs for Linear and Nonlinear Vibrational Spectroscopy

The mid-infrared wavelength range is important for vibrational spectroscopy and trace gas detection, because it contains strong fundamental vibrational transitions. The extension of spectroscopic techniques into the mid-infrared wavelengths is a major area of development. This thesis describes sensitive spectroscopic measurements, which take advantage of the strong mid-infrared transitions to produce novel spectral data. The measurement setups are enhanced using optical frequency combs. Frequency combs are stable laser sources, which emit laser light over a wide optical spectrum. Their development was awarded with one half of the Nobel Prize in Physics in the year 2005. Frequency combs have found many applications in laser spectroscopy, because they combine the coherence and high brightness of a laser source with a wide optical spectrum. The stability of the frequency combs is valuable in high-resolution spectroscopy and metrology. This thesis is focused on two novel measurements: direct frequency comb spectroscopy of radiocarbon methane and double resonance spectroscopy of acetylene. In the former measurement, an optical frequency comb emitting in the mid-infrared region was used as the light source in broadband spectroscopy of radiocarbon methane. The carbon atom in the radiocarbon methane molecule is replaced by the radioactive carbon-14 isotope. High sensitivity was reached by combining a high-power mid-infrared frequency comb with cantilever-enhanced photoacoustic spectroscopy. The photoacoustic detector can take full advantage of the high power spectral density of the frequency comb. This measurement produced the first reported infrared spectrum of radiocarbon methane. In the double resonance spectroscopy measurements, a mid-infrared and a near-infrared light source were simultaneously used to excite two transitions of acetylene, with a shared energy state. This allows detection of spectral lines, which are normally absent in an acetylene infrared spectrum. The method also provides sub-Doppler resolution for determining spectral line positions with high precision. Because of the sub-Doppler resolution, instabilities of the light sources can begin to limit the precision of the measurements. A drastic increase in sensitivity was reached by stabilizing the light sources using frequency combs as references.Keski-infrapuna-alue on tärkeä aallonpituusalue värähdysspektroskopiassa ja hivenkaasuanalyysissä, koska monet molekyylit absorboivat voimakkaasti keski-infrapunasäteilyä. Keski-infrapuna-aallonpituuksilla toimivien menetelmien kehittäminen onkin aktiivinen kehityskohde kaasujen spektroskopiassa. Tämä väitöskirjatyö käsittelee herkkiä spektroskopiamittauksia, joissa hyödynnetään vahvoja keski-infrapunasiirtymiä uuden spektritiedon tuottamiseen. Mittauksissa käytetään moderneja valonlähteitä, kuten optisia taajuuskampoja. Taajuuskammat ovat erityisen vakaita laserlähteitä, jotka emittoivat laajalla aallonpituusalueella tuhansia kapeita laserpiikkejä. Taajuuskammat säilyttävät lasereiden hyvät ominaisuudet, kuten koherenssin ja korkean kirkkauden, mutta tavanomaisista lasereista poiketen, ne tuottavat lasersäteilyä laajalla aallonpituusalueella. Myös taajuuskampojen stabiilius on hyödyllinen ominaisuus monissa mittauksissa, jotka vaativat korkeaa erotuskykyä tai aallonpituuksien tarkkaa tuntemista. Tämä väitöskirja koostuu radiohiilimetaanin taajuuskampaspektroskopiamittauksista ja asetyleenin kaksoisresonanssimittauksista. Radiohiilimetaanin mittauksessa taajuuskampaa käytettiin suoraan valon lähteenä laajakaistaisen spektrin mittaamiseen. Radiohiilimetaanissa molekyylin hiiliatomi on korvattu hiilen radioaktiivisella hiili-14 isotoopilla. Mittauksessa käytettiin valoakustista ilmaisinta korkean herkkyyden saavuttamiseksi hyvin pienellä näytemäärällä. Mittausten tuloksena julkaistiin ensikertaa radiohiilimetaanin infrapunaspektri. Mitatusta spektristä on hyötyä esimerkiksi radiohiilimetaanin optisten ilmaisimien kehityksessä. Kaksoisresonanssikokeissa asetyleeninäytettä säteilytettiin kahden eri aallonpituuden infrapunavalolla, jolloin saatiin viritettyä asetyleenin kaksi siirtymää samanaikaisesti. Menetelmän avulla voidaan havaita siirtymiä, jotka eivät tavallisesti näy asetyleenin infrapunaspektrissä. Menetelmä myös mahdollistaa niin korkean aallonpituustarkkuuden, että käytettyjen laserlähteiden vakaus alkaa rajoittaa mittauksen tarkkuutta. Korkea erotuskyky päästiin kuitenkin hyödyntämään, kun valonlähteet vakautettiin optisen taajuuskamman avulla. Vakautetuilla valonlähteillä saavutettiin myös entistä parempi herkkyys

Ultra-stable microwave generation with a diode-pumped solid-state laser in the 1.5-μm range

We demonstrate the first ultra-stable microwave generation based on a 1.5-μm diode-pumped solid-state laser (DPSSL) frequency comb. Our system relies on optical-to-microwave frequency division from a planar-waveguide external cavity laser referenced to an ultra-stable Fabry-Perot cavity. The evaluation of the microwave signal at ~10GHz uses the transportable ultra-low-instability signal sourceULISS®, which employs a cryo-cooled sapphire oscillator. With the DPSSL comb, we measured −125dBc/Hz phase noise at 1kHz offset frequency, likely limited by the photo-detection shot-noise or by the noise floor of the reference cryo-cooled sapphire oscillator. For comparison, we also generated low-noise microwave using a commercial Er:fiber comb stabilized in similar conditions and observed >20dB lower phase noise in the microwave generated from the DPSSL comb. Our results confirm the high potential of the DPSSL technology for low-noise comb applications