KEYWORDS: Spectrographs, Waveguides, Second harmonic generation, Fabry Perot interferometers, Calibration, Supercontinuum generation, Sum frequency generation, Frequency combs
Next generation extreme precision radial velocity (EPRV) instruments such as the ANDES spectrograph of the Extremely Large Telescope will require an unprecedentedly high-precision calibration approach, particularly in the UB band region in which the most dense stellar absorption lines are present. For this purpose, astrocombs delivering thousands of atomically referenced, evenly-spaced calibration lines across a broad spectrum have the potential to be ideal calibration sources. Here, we report a novel and effective approach to generating a laser frequency comb with a multi- GHz mode spacing covering a broad wavelength range in the UB band. The approach is based on nonlinear mixing between near-infrared ultrafast laser pulses in a MgO:PPLN waveguide. The generated 1-GHz comb, spanning 390–520 nm, was filtered to a 30 GHz sub-comb using a low-dispersion Fabry-Perot etalon. The resultant UB-band astrocomb was then captured on a lab-built cross-dispersion echelle-prism spectrograph, demonstrating well resolved comb lines across the etalon bandwidth of 392–472 nm.
High-resolution echelle spectrographs are critical for modern astronomy. Determining their wavelength solution is a prerequisite calibration, conventionally performed by illuminating the instrument with a broadband hollow-cathode lamp and cross-referencing the resulting two-dimensional spectrum to an emission-line atlas. Challenging requirements for nextgeneration spectrograph calibration are driving the adoption of laser frequency combs in place of lamps. Laser frequency combs offer an exceptional relative calibration scale, but the task of wavelength-tagging individual comb modes currently makes a spectral-line source essential for absolute calibration. Here, we present a new approach that utilizes a laser frequency comb as the sole calibration light source. An ancillary “spectral shaping” spectrograph can excise an individual comb mode and measure its wavelength, which can then be refined to provide sub-fm accuracy wavelength tagging on a proxy astronomical spectrograph echellogram. In a secondary procedure, a comb mode is isolated for each of the echellogram orders in a single measurement, revealing the free spectral range and allowing the order-to-order relationship to be established. These complementary techniques allow the complete calibration of a spectrograph to be achieved using only a laser frequency comb, eliminating reliance on auxiliary light sources and providing direct access to GPS-referenced accuracy.
The instrumentation plan for the ELT foresees the ArmazoNes high Dispersion Echelle Spectrograph (ANDES). The ANDES-project and consortium entered phase B in January 2022 and underwent several (internal and external) revisions by now to ensure that the requirements and eventually the challenging goals can be met by the physical design of the spectrograph.
Among its main scientific goals are the detection of atmospheres of exoplanets and the determination of fundamental physical constants. For this, high radial velocity precision and accuracy are required. Even though the ANDES-spectrograph is designed for maximum intrinsic stability, a calibration and thus a calibration unit is mandatory. To allow for maximum flexibility and modularity the calibration unit is physically split into three calibration units.
We show the design of the calibration units and their individual components, where possible. This includes the electronics, the mechanics, the software supporting and controlling the light guiding and calibration sources.
The Southern African Large Telescope (SALT) is a 10-m class fixed-elevation telescope with a primary mirror composed of 91 spherically figured one metre segments. A prime focus tracker assembly carries the spherical aberration corrector (SAC) and two of SALT’s instruments, SALTICAM (the acquisition and imaging camera) and the multi-purpose Robert Stobie spectrograph (RSS). Included in the tracker payload is a fibre-instrument feed, that positions ~45m long fibre cables coupled to the spectrographs in thermal enclosures beneath the telescope. These are the High-Resolution Spectrograph (HRS) and NIRWALS (Near InfraRed Washburn Astronomical Laboratories Spectrograph). The other major undertaking is a custom-built laser frequency comb and precision radial velocity data pipeline for the HRS, due in 2025. A novel RSS slit-mask IFU was recently commissioned, adding optical IFU spectroscopy to SALT’s capabilities. Work is also underway to develop a new red channel to turn the RSS into a dual-beam spectrograph. A study done in 2021 investigated the feasibility of building deployable robotic arms equipped with mini SACs to take advantage of SALT’s huge uncorrected field of view. Lastly, a pre-study is now underway to explore options for replacing the SAC and prime focus payload on the tracker to improve telescope performance and make provision for future instrument development.
The first generation of ELT instruments includes an optical-infrared high resolution spectrograph, indicated as ELT-HIRES and recently christened ANDES (ArmazoNes high Dispersion Echelle Spectrograph). ANDES consists of three fibre-fed spectrographs ([U]BV, RIZ, YJH) providing a spectral resolution of ∼100,000 with a minimum simultaneous wavelength coverage of 0.4-1.8 μm with the goal of extending it to 0.35-2.4 μm with the addition of an U arm to the BV spectrograph and a separate K band spectrograph. It operates both in seeing- and diffraction-limited conditions and the fibre-feeding allows several, interchangeable observing modes including a single conjugated adaptive optics module and a small diffraction-limited integral field unit in the NIR. Modularity and fibre-feeding allows ANDES to be placed partly on the ELT Nasmyth platform and partly in the Coudé room. ANDES has a wide range of groundbreaking science cases spanning nearly all areas of research in astrophysics and even fundamental physics. Among the top science cases there are the detection of biosignatures from exoplanet atmospheres, finding the fingerprints of the first generation of stars, tests on the stability of Nature’s fundamental couplings, and the direct detection of the cosmic acceleration. The ANDES project is carried forward by a large international consortium, composed of 35 Institutes from 13 countries, forming a team of almost 300 scientists and engineers which include the majority of the scientific and technical expertise in the field that can be found in ESO member states.
The first generation of ELT instruments includes an optical-infrared high resolution spectrograph, indicated as ELT-HIRES and recently christened ANDES (ArmazoNes high Dispersion Echelle Spectrograph). ANDES consists of three fibre-fed spectrographs (UBV, RIZ, YJH) providing a spectral resolution of ∼100,000 with a minimum simultaneous wavelength coverage of 0.4-1.8 µm with the goal of extending it to 0.35-2.4 µm with the addition of a K band spectrograph. It operates both in seeing- and diffraction-limited conditions and the fibre-feeding allows several, interchangeable observing modes including a single conjugated adaptive optics module and a small diffraction-limited integral field unit in the NIR. Its modularity will ensure that ANDES can be placed entirely on the ELT Nasmyth platform, if enough mass and volume is available, or partly in the Coudé room. ANDES has a wide range of groundbreaking science cases spanning nearly all areas of research in astrophysics and even fundamental physics. Among the top science cases there are the detection of biosignatures from exoplanet atmospheres, finding the fingerprints of the first generation of stars, tests on the stability of Nature’s fundamental couplings, and the direct detection of the cosmic acceleration. The ANDES project is carried forward by a large international consortium, composed of 35 Institutes from 13 countries, forming a team of more than 200 scientists and engineers which represent the majority of the scientific and technical expertise in the field among ESO member states.
The Southern African Large Telescope (SALT) is developing precision radial velocity capability for its high-resolution spectrograph (HRS). The instrument's high-stability (HS) mode includes a fibre double scrambler and makes provision for simultaneous thorium-argon (ThAr) injection into the calibration fibre. Given the limitations associated with ThAr lamps, as well as the cost and complexity of turn-key commercial laser frequency combs (LFCs), we are in the process of designing and building a bespoke LFC for the Red channel of the HRS (555-890 nm). At a later stage we plan to extend the wavelength range of the LFC to include parts of the blue channel (370-555 nm) as well. A data reduction pipeline capable of delivering precision radial velocity results for the HS mode is also currently under development. We aim to have the LFC and PRV pipeline available for science operations in early 2024.
With this work, we present a new class of diode pumped solid-state based optical frequency comb (OFC) technology based on a Kerr-lens modelocked oscillator, which possesses robust self-starting capabilities. By combining this novel technology together with Airbus’ capabilities of building highly stable, bonded optical space instruments, a preliminary optics module is demonstrated that produces sub-150-fs pulses with a repetition rate of 1.51 GHz, an output power of 12 mW and a central wavelength of 1078 nm with a bandwidth of 9.1 nm. This novel technology is presented for the first time to provide some preliminary results and to show the capability of the technology. Operation has so far been demonstrated for tens of hours without performance degradation and it is expected to be significantly less sensitive against radiation than fibre based OFCs. All these features are packed in a mechanically robust and small micro-optical package, with the clear aim to deliver a compact solution fully suitable for space applications.
Supercontinuum generation in bulk media is not normally observed at the nJ-level pulse energies available from high-repetition-rate femtosecond oscillators. Here, we present results demonstrating how a visible supercontinuum can be produced in bulk orientation-patterned gallium phosphide from 100-MHz 1040-nm femtosecond pulses with energies of up to 32 nJ. High-order parametric gain near 550 nm, seeded by self-phase-modulated spectral sidebands, underpins this new and simple supercontinuum process which yields an output spectrum spanning from the blue/green to the red.
Here we report for the first time nonlinear frequency conversion in OP-GaP layers grown by hydride vapor-phase epitaxy on OP-GaAs templates. Multi-grating 3-inch wafer design enabled discrete wavelength tuning via stepped gratings, continuous tuning via fan gratings, and bandwidth engineering via chirped gratings, with 14- 35.2-micron periods that propagated up to 300 microns in a 1.2-mm-thick layer before breakdown. Polished, AR-coated, 3-mm-long OPO crystals were fabricated and pumped at 1040 nm (5.5W, 100 MHz, 2.5 ps) with a Chromacity Yb-fiber laser, yielding output powers of 140, 90, and 60 mW at idler wavelengths of 5.6, 7.8, and 10.7 microns respectively
We demonstrate path-integrated simultaneous concentration measurements of water, methane and ethane, measuring spectra across the 3.1–3.5-μm range using 0.05 cm-1 resolution Fourier-transform spectroscopy in-line with an ultrafast optical parametric oscillator and a simple, non-compliant target. Illumination spectra were extracted from a fitting procedure which simultaneously minimized the rms error between the experimental spectrum and a synthetic spectrum calculated from the envelope and a fitted mixture of PNNL or HITRAN absorbance data for water, methane and ethane. Simultaneous methane, ethane and water measurement at 30-m range were initially performed. Indoor measurements launched light from the OPO through a 20-cm-long gas cell containing a 1.5±0.15% ethane-in-air mixture. Light was reflected from a rough Al-foil target. Best-fit concentrations were determined to be 1.15% (water), 1860 ppb (methane) and 1.37 % (ethane). The methane background value is consistent with reported ambient levels. Respective water and ethane values were consistent with the ambient relative humidity. The second experiment demonstrated real-time methane emission measurement at 70-m range. A 2% methane:air mix was released for 100 seconds at a rate of 103 μgs-1 at a distance of 65 m from the OPO. The signal was recorded from a simple target of rough aluminum foil situated 70 m from the OPO, with the beam passing near the emission point. This work demonstrates our ability to extract concentration data from a single spectrum with no need for averaging, which provides a real-time and quantitative monitoring capability.
High resolution spectroscopy enables the detection of atmospheres of exoplanets. To reach the required radial velocity precision of about 1 m/s, calibration with even more precise sources is mandatory. HIRES will employ several calibration sources, the most important ones are an Laser Frequency Comb (LFC) and Fabry-P´erots (FP). The LFC needs to be filtered with a set of FP. One possible solution is to illuminate this set of FP with a broadband light source and use them as calibrators, when they are not used for filtering the LFC. It has been demonstrated that passively-stabilized FP can perform better than 10 cm/s per night. We give an overview of the currently used FP in different surveys and compare their individual features. For the FP which may be used in HIRES we discuss different configuration. We show that the Finesse and FSR of the FP needs to be optimized with regard to the resolution of the spectrograph and we outline how we aim to fulfill the requirements of HIRES.
The broad and slowly varying spectral features of liquids and solids require broadband sources in the mid- to long-wave infrared for their detection and identification. We present here a range of measurements made using uniquely tunable femtosecond optical parametric oscillators, which have enabled stand-off Fourier-transform spectroscopy to be implemented across a large part of the spectral fingerprint region. In this way we have achieved active stand-off detection of liquids on surfaces, of powders and of airborne liquid particles in aerosol form. We discuss the optical parametric oscillator technology, the spectroscopy implementations and the detection capabilities and limitations of the techniques.
Optical parametric oscillators (OPOs), pumped by Ti:sapphire and Yb-doped femtosecond lasers, provide unique capabilities to address a broad range of parameters of interest to precision spectroscopy. We review here a variety of OPOs under development that offer tuning from 1.5 to 13 μm, repetition rates from 100 MHz to 10 GHz and pulse durations from < 25 fs to a few picoseconds. Spectroscopic techniques revealing the individual frequency comb modes are discussed, along with dual-comb spectroscopy at 3 μm and from 6-8 μm.
Precise astronomical spectroscopy with the forthcoming E-ELT and its high resolution spectrograph HIRES will address a number of important science cases,1 e.g. detection of atmospheres of exoplanets. Challenging technical requirements have been identified to achieve these cases, principal among which is the goal to achieve a radial velocity precision on the order of 10 cms-1. HIRES will experience systematic errors like intrapixel variations and random variations like fiber noise, caused by the non-uniform illumination of the coupling fibers, with these and other systematic errors affecting the performance of the spectrograph. Here, we describe the requirements for the calibration sources which may be used for mitigating such systematic errors in HIRES. Precise wavelength calibration with wide-mode-spacing laser frequency combs (LFCs), so called astrocombs, has been demonstrated with different astronomical spectrographs. Here we present a comparison of currently used astrocombs and outline a possible solution to meet the requirements of HIRES with a single broadband astrocomb.
The instrumentation plan for the E-ELT foresees a High Resolution Spectrograph (HIRES). Among its main goals are the detection of atmospheres of exoplanets and the determination of fundamental physical constants. For this, high radial velocity precision and accuracy are required. HIRES will be designed for maximum intrinsic stability. Systematic errors from effects like intrapixel variations or random errors like fiber noise need to be calibrated. Based on the main requirements for the calibration of HIRES, we discuss different potential calibration sources and how they can be applied. We outline the frequency calibration concept for HIRES using these sources.
We report a femtosecond optical parametric oscillator (OPO) based on the new semiconductor gain material orientation patterned gallium phosphide (OP-GaP) and being the first example of a broadband OPO operating across the molecular fingerprint region. OP-GaP crystals with lengths of 1 mm and several patterning periods were diced, polished, and antireflection (AR) coated for near- to mid-infrared wavelengths. We configured a synchronously pumped OP-GaP OPO in a 101.2-MHz resonator with high reflectivity from 1.15–1.35 μm, pumped with 150-fs pulses from a 1040-nm femtosecond laser (Chromacity Spark). The coating of one spherical mirror was optimized for transmission at the pump wavelength of 1040 nm and for high reflectivity at the resonant signal wavelength in a range from 1.15–1.35 μm, while the other spherical mirror collimated the idler beam emerging from the OP-GaP crystal and was silver coated to provide high reflectivity for all idler wavelengths. This collimated idler beam was output-coupled from the cavity by transmission through a plane mirror coated with high transmission for the idler wavelengths (5–12 μm) and high reflectivity for the signal wavelengths (1.15–1.35 μm) on an infrared-transparent ZnSe substrate. Idler spectra centered from 5.4–11.8 μm and extending to 12.5 μm were collected. The maximum average power was 55 mW at 5.4 μm with 7.5 mW being recorded at 11.8 μm. Details of Fourier transform spectroscopy using water vapor and a polystyrene reference standard are presented.
Lasers developed for defence related applications typically encounter issues with reliability and meeting desired specification when taken from the lab to the product line. In particular the harsh environmental conditions a laser has to endure can lead to difficulties. This paper examines a specific class of laser, namely actively mode-locked fibre lasers (AMLFLs), and discusses the impact of environmental perturbations. Theoretical and experimental results have assisted in developing techniques to improve the stability of a mode-locked pulse train for continuous operation. Many of the lessons learned in this research are applicable to a much broader category of lasers.
The AMLFL consists of a fibre ring cavity containing a semiconductor optical amplifier (SOA), an isolator, an output coupler, a circulator, a bandpass filter and a modulator. The laser produces a train of 6-ps pulses at 800 nm with a repetition rate in the GHz regime and a low-noise profile. This performance is realisable in a laboratory environment. However, even small changes in temperature on the order of 0.1 °C can cause a collapse of mode-locked dynamics such that the required stability cannot be achieved without suitable feedback. Investigations into the root causes of this failure were performed by changing the temperature of components that constitute the laser resonator and observing their properties.
Several different feedback mechanisms have been investigated to improve laser stability in an environment with dynamic temperature changes. Active cavity length control will be discussed along with DC bias control of the Mach-Zehnder modulator (MZM).
The spectrum of mid-infrared light scattered from an actively illuminated aerosol was used to distinguish between different chemicals. Using spectrally broad illumination from an optical parametric oscillator covering 3.2 – 3.55 μm, characteristic absorption features of two different chemicals were detected, and two similar molecules were clearly distinguished using the spectra of backscattered light from each chemical aerosol.
In this work we present a method used to study the spherical and chromatic aberrations contribution near the focal point of a refractive optical system. The actual focal position is measured by scanning a pinhole attached on the front of a power detector, which are scanned along the optical axis using a motorized stage with 1 μm resolution. Spherical aberration contribution was analyzed by changing the pupil aperture, by modifying the size of the input iris diaphragm and for each case, measuring the actual laser power vs the detector position. Chromatic aberration was analyzed by performing the same procedure but in this case we used an ultra-broad-band femtosecond laser. The results between ML and CW operation were compare. Experimental results are presented.
The first generation of E-ELT instruments will include an optic-infrared High Resolution Spectrograph, conventionally indicated as EELT-HIRES, which will be capable of providing unique breakthroughs in the fields of exoplanets, star and planet formation, physics and evolution of stars and galaxies, cosmology and fundamental physics. A 2-year long phase A study for EELT-HIRES has just started and will be performed by a consortium composed of institutes and organisations from Brazil, Chile, Denmark, France, Germany, Italy, Poland, Portugal, Spain, Sweden, Switzerland and United Kingdom. In this paper we describe the science goals and the preliminary technical concept for EELT-HIRES which will be developed during the phase A, as well as its planned development and consortium organisation during the study.
We present preliminary results of the commissioning and testing of SALT-CRISP (SALT-Calibration Ruler for Increased Spectrograph Precision), a Laser Frequency Comb (LFC) built by Heriot-Watt University and temporarily installed at the Southern African Large Telescope (SALT). The comb feeds the High Stability mode of SALT's High Resolution Spectrograph (HRS) and fully covers the wavelength range of the red channel of the HRS: 555-890 nm. The LFC provides significantly improved wavelength calibration compared to a standard Thorium-Argon (ThAr) lamp and hence offers unprecedented opportunities to characterise the resolution, stability and radial velocity precision of the HRS. Results from this field trial will be incorporated into subsequent LFC designs.
A hyperspectral imaging system was implemented using active illumination in the 3-4-μm band from an MgO:PPLN
ultrafast optical parametric oscillator. Using a staring configuration based on a high-resolution mid-IR camera it was
possible to distinguish between liquid chemicals based on their absorption characteristics, demonstrating the potential for
standoff detection of a wide range of liquids.
We discuss recent advances in the stabilization and application of femtosecond frequency combs based on optical parametric oscillators (OPOs) pumped by femtosecond lasers at 800 and 1060 nm. A method for locking to zero the carrier-envelope-offset of a Ti:sapphire-pumped OPO comb is described. The application of Yb:KYW-laser-pumped dual-combs for mid-infrared spectroscopy is detailed, specifically methane spectroscopy at approximately a 0.7% concentration at 1 atm.
We present the first demonstration of stand-off Fourier transform infrared spectroscopy using a broadband mid-infrared femtosecond optical parametric oscillator, with spectral coverage over 2700–3200 cm-1. Remote spectroscopy and chemical detection from 2700–3100 cm-1 is demonstrated for a thiodiglycol drop on concrete and anodized aluminum surfaces at a stand-off distance of 2 meters, as well as open-path spectroscopy of atmospheric water vapor from a concrete target at the same range. Comparison of the measured stand-off spectra with archived reference spectra for thiodiglycol and water vapor show good agreement. This technique provides greater spatial coherence and spectral brightness than a thermal source, and wider spectral coverage than a typical quantum-cascade laser, thereby presenting opportunities for application in the detection of industrial pollutants and the environmental identification of chemical warfare agents, explosives or other hazardous materials.
We demonstrated coherent pulse synthesis between the carrier-envelope phase slip (CEPS) locked second-harmonic
(SH) pulses from an optical parametric oscillator (OPO) and those from its pump laser. By using a single nonlinear
crystal with cascaded gratings for parametric and SH generation, we maximized the common-mode rejection of
environmental noise, obtaining a temporal overlap between the pulses as low as 30 attoseconds in an observation time of
20 ms. The CEPS frequencies of the pump laser and the OPO SH signal were locked individually to the same subharmonic
of the repetition rate with a coherence time of at least 1.4 ms by using the pump supercontinuum as a common
reference. Auto-correlation traces of the combined pulses showed an 8:1 ratio between the peak and the background once
the CEPS frequencies were locked, in contrast with a much lower ratio when they were not locked, indicating successful
pulse synthesis. This research illustrates the viability of using OPOs for sub-femtosecond optical pulse synthesis. The
very low timing jitter and phase coherence between the pulses from this system, which spans from the ultraviolet (SH of
the pump) to mid-infrared (idler), also make the system a powerful tool for optical spectroscopy and optical metrology.
The combination of high spatial coherence, wide tunability and broad intrinsic bandwidth makes femtosecond optical
parametric oscillators (OPOs) uniquely attractive sources for spectroscopy in the visible and infrared. In the mid-infrared
the idler pulse bandwidths from such systems can extend over several hundred nanometres, making Fourier-transform
spectroscopy possible, and transferring the wavelength calibration and resolution constraints from the OPO to
the detection system. Unlike thermal sources of mid-infrared radiation, the spatial coherence of the output from
femtosecond OPOs is extremely high, with the potential for spectroscopic measurements to be made over long free-space
path lengths, in fiber or at the focus of a microscope objective. Using OPOs based on MgO:PPLN, and pumped by a
self-modelocked Ti:sapphire laser, we have shown free-space and photonic-crystal-fiber-based spectroscopy of methane
to concentrations as low as 50 ppm. The spectral bandwidth of the idler pulses used for gas sensing exceeds 200 nm,
allowing the principal methane absorption lines around 3.3 μm to be acquired without wavelength tuning the OPO.
Practical Ti:sapphire and Yb:fiber pumped based OPOs have been demonstrated that offer combinations of flexible
tuning, high stability, low-threshold operation and high-energy output pulses.
Solid immersion lens (SIL) microscopy combines the advantages of conventional microscopy with those of near-field techniques, and is being increasingly adopted across a diverse range of technologies and applications. A comprehensive overview of the state-of-the-art in this rapidly expanding subject is therefore increasingly relevant. Important benefits are enabled by SIL-focusing, including an improved lateral and axial spatial profiling resolution when a SIL is used in laser-scanning microscopy or excitation, and an improved collection efficiency when a SIL is used in a light-collection mode, for example in fluorescence micro-spectroscopy. These advantages arise from the increase in numerical aperture (NA) that is provided by a SIL. Other SIL-enhanced improvements, for example spherical-aberration-free sub-surface imaging, are a fundamental consequence of the aplanatic imaging condition that results from the spherical geometry of the SIL. Beginning with an introduction to the theory of SIL imaging, the unique properties of SILs are exposed to provide advantages in applications involving the interrogation of photonic and electronic nanostructures. Such applications range from the sub-surface examination of the complex three-dimensional microstructures fabricated in silicon integrated circuits, to quantum photoluminescence and transmission measurements in semiconductor quantum dot nanostructures.
This paper reports the imaging of a silicon flip-chip with high resolution by detection of the photocurrent generated by the two-photon absorption of 1530nm light from a femtosecond Er:fiber laser. High resolution imaging was made possible by the inclusion of a silicon solid immersion lens, which increased the numerical aperture of the microscope. Using this technique, features on a sub-micron scale are clearly resolvable with excellent contrast, and the resolution of the system was found to be 325nm.
We report the use of electric field induced second harmonic generation to probe electrical signals in a CMOS chip. The second harmonic of incident 2.3μm illumination provided by a femtosecond optical parametric oscillator was measured and shown to depend quadratically on both optical intensity and on the applied DC electric field. By using a near infrared photomultiplier tube it was possible to monitor directly the electrical waveform in the chip on the oscilloscope.
Laser ranging and burst illumination imaging (BIL) in the atmospheric transmission window at 1.5μm are made difficult by speckle effects which are observed when a rough surface is illuminated by a laser beam with a coherence length greater than the characteristic surface feature size. For the narrowband pulsed lasers currently used, this length is of the order of a few millimetres which leads to observable speckle effects for many common surfaces. In this context we describe progress towards the development a short-coherence length laser source operating at 1.5μm and based on optical parametric amplification of broadband seed pulses from a modelocked femtosecond erbium-doped fibre laser.
Our optical parametric amplifier (OPA) system comprises a compact actively Q-switched 1047nm Nd:YLF laser, operating at ~1kHz repetition frequency, to which a 54MHz femtosecond 1.55μm Er:fibre laser is synchronised. The fibre laser produces bandwidth-limited 100fs pulses which are stretched by chromatic dispersion in a spool of SMF28 fibre to match the 3.5ns duration of the Q-switched pulses. Pulses from the Nd:YLF and Er:fibre lasers act as the pump and seed respectively for an OPA based on an aperiodically-poled crystal of MgO:PPLN containing a single linearly chirped grating. The chirp grating enables broadband parametric amplification across a wavelength range comparable with the spectral bandwidth of the seed pulses, amounting to ~150nm in the wings of the spectrum. Early results from this system have demonstrated output energies of 2.55μJ and a single-pass gain ~51dB and are expected to be increased with continued development of the project.
Conventionally optical parametric oscillators (OPOs) have been used in high-resolution absorption-spectroscopy as narrow-band tuneable sources where the measurement resolution is determined by the OPO output linewidth, rather than the wavelength resolution of the detector. In contrast, the absorption spectroscopy of gases and other media has for many years been carried out using instruments such as Fourier-transform infrared (FTIR) spectrometers or high-resolution diffraction-grating-based tuneable monochromators. These techniques commonly utilise broadband thermal sources with highly-divergent illumination beams limiting their use in remote sensing or fibre delivery applications.
The work presented here reports a new approach to FTIR spectroscopy based around a novel Ti:sapphire pumped, signal-resonant OPO that uses a 10mm crystal of aperiodically-poled lithium niobate (APPLN) as the gain medium producing an idler output covering a 3.2-3.85μm tuning range with a typical full-width-half-maximum bandwidth of 85nm. Methane was used to demonstrate the technique since the OPO tuning range almost completely covers the strongest mid-infrared absorption lines of methane from 3.0-3.7μm (limited only by the available resonator optics). A double-beam Michelson interferometer was built around the OPO idler beam using a helium-neon laser as the second beam to self-calibrate each trace. Course tuning of the OPO resulted in the measurement of absorption data across the 3.2-3.85μm tuning range using methane held at pressures ranging from 2000mbar down to 25mbar. A maximum resolution of around 1cm-1 was achieved using a simple rapidly scanning mirror assembly indicating that with further development this approach could yield very high-resolution measurements.
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