Mid-Infrared Optical Frequency Combs based on Difference Frequency Generation for Molecular Spectroscopy

Mid-infrared femtosecond optical frequency combs were produced by difference frequency generation of the spectral components of a near-infrared comb in a 3-mm-long MgO:PPLN crystal. We observe strong pump depletion and 9.3 dB parametric gain in the 1.5 \mu m signal, which yields powers above 500 mW (3 \mu W/mode) in the idler with spectra covering 2.8 \mu m to 3.5 \mu m. Potential for broadband, high-resolution molecular spectroscopy is demonstrated by absorption spectra and interferograms obtained by heterodyning two combs.Comment: 11 pages, 8 figure

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

Frequency comb metrology with an optical parametric oscillator

We report on the first demonstration of absolute frequency comb metrology with an optical parametric oscillator (OPO) frequency comb. The synchronously-pumped OPO operated in the 1.5-μm spectral region and was referenced to an H-maser atomic clock. Using different techniques, we thoroughly characterized the frequency noise power spectral density (PSD) of the repetition rate frep, of the carrier-envelope offset frequency fCEO, and of an optical comb line νN. The comb mode optical linewidth at 1557 nm was determined to be ~70 kHz for an observation time of 1 s from the measured frequency noise PSD, and was limited by the stability of the microwave frequency standard available for the stabilization of the comb repetition rate. We achieved a tight lock of the carrier envelope offset frequency with only ~300 mrad residual integrated phase noise, which makes its contribution to the optical linewidth negligible. The OPO comb was used to measure the absolute optical frequency of a near-infrared laser whose second-harmonic component was locked to the F = 2→3 transition of the 87Rb D2 line at 780 nm, leading to a measured transition frequency of νRb = 384,228,115,346 ± 16 kHz. We performed the same measurement with a commercial fiber-laser comb operating in the 1.5-μm region. Both the OPO comb and the commercial fiber comb achieved similar performance. The measurement accuracy was limited by interferometric noise in the fibered setup of the Rb-stabilized laser

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

Fiber laser system for the detection of trace gas

The research presented in this thesis is on the development of a trace gas detection system based on intracavity absorption spectroscopy (ICAS). The developed system was capable of detecting nitrous oxide (N2O) and acetylene (C2H2) gas at sub-ppmv levels. The P (12) rotational line of N2O at ~1522.20 nm, and the R (5) and R (4) rotational lines of C2H2 at ~1522.22 nm were used for detection. A fiber Bragg grating was incorporated into the ICAS cavity to enhance the system sensitivity by allowing the selection of strong absorption lines for detection. By operating the fiber laser based ICAS system at threshold, multiple circulations of the amplified spontaneous emission inside the cavity enhanced the system detection sensitivity. Further, the laser wavelength contained multi-longitudinal modes, which improved the system sensitivity. The system sensitivity was explored for two intra-cavity gas cells: a multi-pass Herriott cell and a gas cell based on a hollow-core photonic crystal fiber (HC-PCF). The system operated at room temperature and a polarization-maintaining erbium-doped fiber was used as a saturable absorber to help eliminate mode hopping in the laser cavity

Erbium-doped-fiber-based broad visible range frequency comb with a 30 GHz mode spacing for astronomical applications

Optical frequency combs have the potential to improve the precision of the radial velocity measurement of celestial bodies, leading to breakthroughs in such fields as exoplanet exploration. For these purposes, the comb must have a broad spectral coverage in the visible wavelength region, a wide mode spacing that can be resolved with a high dispersion spectrograph, and sufficient robustness to operate for long periods even in remote locations. We have realized a comb system with a 30 GHz mode spacing, 62 % available wavelength coverage in the visible region, and 40 dB spectral contrast by combining a robust erbium-doped-fiber-based femtosecond laser, mode filtering with newly designed optical cavities, and broadband-visible-range comb generation using a chirped periodically-poled LiNbO3 ridge waveguide. The system durability and reliability are also promising because of the stable spectrum, which is due to the use of almost all polarization-maintaining fiber optics, moderate optical power, and good frequency repeatability obtained with a wavelength-stabilized laser.Comment: 19 pages, 5 figure

The optical frequency comb fibre spectrometer

Optical frequency comb sources provide thousands of precise and accurate optical lines in a single device enabling the broadband and high-speed detection required in many applications. A main challenge is to parallelize the detection over the widest possible band while bringing the resolution to the single comb-line level. Here we propose a solution based on the combination of a frequency comb source and a fibre spectrometer, exploiting all-fibre technology. Our system allows for simultaneous measurement of 500 isolated comb lines over a span of 0.12 THz in a single acquisition; arbitrarily larger span are demonstrated (3,500 comb lines over 0.85 THz) by doing sequential acquisitions. The potential for precision measurements is proved by spectroscopy of acetylene at 1.53 μm. Being based on all-fibre technology, our system is inherently low-cost, lightweight and may lead to the development of a new class of broadband high-resolution spectrometers

Accurate mesurements of spectroscopic line parameters of atmospheric relevant molecules

2015 - 2016This thesis deals with the development of frequency-stabilized laser absorption spectrometers in the near-infrared to perform precision measurements on molecular spectra of atmospheric interest, such as acetylene and carbon dioxide. Two novel schemes have been implemented: the former is based on a pair of phase-locked diode lasers, one of them being a reference oscillator at 1.4 micron; the latter makes use of a self-referenced optical frequency comb synthesizer. In the second apparatus, a diode laser at a wavelength of 2 micron is frequency locked to the nearest tooth of the comb. In both cases, an absolute frequency scale is produced underneath any absorption spectrum. Line intensity factors and line widths have been determined with unprecedented accuracy. For carbon dioxide, our measurements give an important experimental test of ab-initio calculations, which have been recently performed at University College London by Jonathan Tennyson and coworkers. [edited by author]XXIX n.s