High‐resolution narrowband CARS spectroscopy in the spectral fingerprint region

Coherent anti‐Stokes Raman scattering (CARS) spectroscopy is an important technique for spectroscopy and chemically selective microscopy, but wider implementation requires dedicated versatile tunable sources. We describe an optical parametric oscillator (OPO) based on a magnesium oxide‐doped periodically poled lithium niobate crystal, with a novel variable output coupler, used as a tunable coherent light source. The OPO's signal wavelength ranges from 880 to 1040 nm and its idler wavelength from 1090 to 1350 nm. We use this OPO to demonstrate high‐resolution narrowband CARS spectroscopy on bulk polystyrene from 900 to 3600 cm⁻¹, covering a large part of the molecular fingerprint region. Recording vibrational spectra using narrowband CARS spectroscopy has several advantages over spontaneous Raman spectroscopy, which we discuss. We isolate the resonant part of the CARS spectrum and compare it to the spontaneous Raman spectrum of polystyrene using the maximum entropy method of phase retrieval; we find them to be in extremely good agreement. Copyright © 2009 John Wiley & Sons, Ltd.     

Broadband (1000 cm−1) multiplex CARS spectroscopy: Application to polarization sensitive and time-resolved measurements

A multiplex CARS spectrometer based on a cw diode-pumpedQ-switched Nd: YLF laser, a broadband dye-laser and a multichannel spectrum detection system has been constructed. Excellent mode characteristics of the laser beams and high pulse repetition rate (2 kHz) have resulted in good signal-to-noise ratio achieved with a few seconds accumulation time. A 1000 cm^–1 wide spectral range is covered in a single CARS spectrum with an expanded bandwidth of the dye laser. A thin-jet sampling method is used in order to avoid the phase-matching limitation. The efficient spectral intensity normalization by the reference (CCl₄) nonresonant spectrum and subsequent computer fitting have been implemented. The performance of the system is demonstrated by two different experiments. First, the polarization sensitive measurements (PS-CARS) of cyclohexane show its potential for accurate Raman depolarization ratio determination and for detection of weak (overlapped) Raman bands. Second, the transient resonance CARS measurement of the lowest excited triplet state of all-trans retinal indicate its feasibility to time-resolved CARS spectroscopy of fluorescent excited states.     

Bismuth triborate (BiB3O6) optical parametric oscillators

We report the phase matching of parametric frequency conversion in the nonlinear material BiB₃O₆ (BiBO) and on an investigation of optical parametric oscillators (OPOs) of this new crystal. Based on the calculation of collinear type I and type II phase matching within the refractive-index planes, the most favorable directions for phase matching are identified for OPOs pumped by the fundamental or the harmonics of 1064-nm Nd-doped lasers. Based on these results, pulsed 532-nm-pumped ns OPOs are realized. The pump source is either a Q-switched high repetition rate (10 kHz) Nd:YVO₄ laser (with a pulse energy of 24 J) or a low repetition rate (10 Hz), high pulse energy (120 mJ) Nd:YAG laser system. The BiBO OPO pumped by the Nd:YVO₄ laser showed a very low threshold of 0.047 J/cm². At an average pump power of 2.4 W the total OPO output power was 630 mW. By changing the phase-matching angle within the yz plane from 0 to 11.6° the signal wavelength was tuned from 735 nm to 970 nm, while the spectral width changed from 0.2 nm to 1.4 nm. By pumping the OPO with the Nd:YAG laser, the OPO had a threshold of 0.12 J/cm², a steep slope (59%) and a high total efficiency (of up to 48%). Due to divergence broadening the spectral width changes from 8.5 nm at 800 nm to 70 nm near degeneracy. The properties of BiBO determined from the experimental results are compared with those of well-known nonlinear materials such as BBO, LBO and KTP. PACS 42.65.-k; 42.65.Yj; 42.70.-a; 42.70.Mp     

Differential two-signal picosecond-pulse coherent anti-Stokes Raman scattering imaging microscopy by using a dual-mode optical parametric oscillator

We propose and demonstrate a novel differential two-signal technique of coherent anti-Stokes Raman scattering (CARS) imaging microscopy using a picosecond (ps) optical parametric oscillator (OPO). By adjusting a Lyot filter inside the cavity, we operated the OPO oscillating in two stable modes separated by a few nanometers. The CARS images generated by the two modes are separated by a spectrograph behind the microscope setup, and their differential image is directly obtained by balanced lock-in detection. The feasibility of the technique is experimentally verified by imaging micrometer-sized polystyrene beads immersed in water.     

Fourier transform spectral interferometric coherent anti-Stokes Raman scattering (FTSI-CARS) spectroscopy

A novel Fourier transform spectral interferometric (FTSI) multiplex coherent anti-Stokes Raman scattering (CARS) technique is developed to extract the vibrational spectrum equivalent to the spontaneous Raman scattering. The conventional FTSI method is modified to use the internal nonresonant CARS signal as a local oscillator to perform spectral interferometry. Utilizing the causality of the coherent vibration (i.e., there should be no signal before the laser excitation), this new FTSI method recovers the entire complex vibrational spectral parameters. We demonstrate this technique with a previously reported single-pulse multiplex CARS method that uses a single phase-controlled broadband ultrafast laser pulse.     

Compact fibre-based coherent anti-Stokes Raman scattering spectroscopy and interferometric coherent anti-Stokes Raman scattering from a single femtosecond fibre-laser oscillator

We demonstrate a new approach to CARS spectroscopy by efficiently synthesizing synchronized narrow-bandwidth (less than 10 cm⁻¹) pump and Stokes pulses (frequency difference continuously tunable upto ≈3000 cm⁻¹) based on spectral compression together with second harmonic generation (in periodically-poled nonlinear crystals) of femtosecond pulses emitted by a single compact Er-fibre oscillator. For a far better signal to non-resonant background contrast, interferometric CARS (I-CARS) is demonstrated and CARS signal enhancement upto three orders of magnitude is achieved by constructive interference with an auxiliary local oscillator at anti-Stokes field, also synthesized by spectral compression of pulses emitted from the same fibre oscillator.     

Single‐beam coherent anti‐Stokes Raman scattering (CARS) spectroscopy of gas‐phase CO2 via phase and polarization shaping of a broadband continuum

Coherent anti‐Stokes Raman scattering (CARS) spectroscopy of gas‐phase CO₂ is demonstrated using a single femtosecond (fs) laser beam. A shaped ultrashort laser pulse with a transform‐limited temporal width of ∼7 fs and spectral bandwidth of ∼225 nm (∼3500 cm⁻¹) is employed for simultaneous excitation of the CO₂ Fermi dyads at ∼1285 and ∼1388 cm⁻¹. CARS signal intensities for the two Raman transitions and their ratio as a function of pressure are presented. The signal‐to‐noise ratio of the single beam–generated CO₂ CARS signal is sufficient to perform concentration measurements at a rate of 1 kHz. The implications of these experiments for measuring CO₂ concentrations and rapid pressure fluctuations in hypersonic and detonation‐based chemically reacting flows are also discussed. Copyright © 2010 John Wiley & Sons, Ltd.     

Ultrabroadband single-pulse CARS of liquids using a spatially dispersive Stokes beam

A double-channel spectrometer, which enables to acquire ultrabroadband single-pulse spectra of liquids by Coherent Anti-Stokes Raman Spectroscopy (CARS), is described. The method used to fulfill the phase-matching condition is based on the fact that the CARS efficiency in dispersive media is the largest when the interactive waves cross each other under frequency-determined angles. The dependence of the spatial separation between the pump and Stokes beam, in front of the crossing CARS lens, due to their frequency difference is analysed. It is shown that the different spectral components of an ultrabroadband Stokes source have phase-matched the CARS process when they are laterally shifted by a conjugated prism pair and focused into the sample. The method is tested in the spectral region 2800–3800 cm^–1 of a non-resonant medium (CCl₄) using an ultrabroadband dye laser (1000 cm^–1 FWHM). The influence of the Stokes beam spatial dispersion on the width of CARS generation is demonstrated. By this method, 1060 cm^–1 wide single-pulse spectra of the OH stretching vibration of liquid water are obtained for the first time. The ratio between the resonant and non-resonant part of the third-order susceptibility in water and methanol is determined.     

Heterodyne interferometric polarization coherent anti‐Stokes Raman scattering (HIP‐CARS) spectroscopy

We demonstrate a new technique that combines polarization sensitivity of the coherent anti‐Stokes Raman scattering (CARS) response with heterodyne amplification for background‐free detection of CARS signals. In this heterodyne interferometric polarization CARS (HIP‐CARS), the major drawbacks of polarization and heterodyne CARS are rectified. Using a home‐built picosecond optical parametric oscillator, we are able to address vibrational stretches between 600 and 1650 cm⁻¹ and record continuous high‐resolution Raman equivalent HIP‐CARS spectra. Copyright © 2010 John Wiley & Sons, Ltd.     

Frequency-stable operation of a diode-pumped continuous-wave RbTiOAsO(4) optical parametric oscillator

Frequency-stable operation of a diode-pumped continuous-wave optical parametric oscillator (OPO) of RbTiOAsO(4) is demonstrated. Piezoelectric and fast electro-optic control of the optical length of the two-mirror OPO cavity (resonant for the pump and the idler waves) compensates for thermal changes in the refractive index of the OPO crystal (induced by absorption of pump light) and acoustic perturbations of the cavity length. Pumped by 405mW of the 810-nm output of a GaAlAs masterf-oscillator-tapered-amplifier diode laser system, the OPO generates a power-stable single-frequency signal wave at 1.24microm with an output of 84mW and a spectral bandwidth of less than 10MHz.     

Electro-optic spectral tuning in a continuous-wave, asymmetric-duty-cycle, periodically poled LiNbO(3) optical parametric oscillator

We demonstrate electro-optic spectral tuning in a continuous-wave periodically poled LiNbO(3) (PPLN) optical parametric oscillator (OPO). We achieve 8.91 cm(-1) of rapid spectral tuning, with a linear tuning rate of 2.89 cm(-1) /(kV/mm), by applying electric fields up to +/-1.5 kV/mm across the crystal while it is operating within the OPO. Intentionally poling the PPLN crystal with an asymmetric domain structure enables tuning, and numerical predictions closely match the experimental observations. The tuning is considerably larger than the typical operational bandwidth of the OPO, indicating that we are in fact shifting the gain curve of the PPLN crystal.     

Five‐cycle pulses near λ = 3 μm produced in a subharmonic optical parametric oscillator via fine dispersion management

Five‐cycle (50 fs) mid‐IR pulses at 80‐MHz repetition rate are produced using a degenerate (subharmonic) optical parametric oscillator (OPO), synchronously pumped by an ultrafast 1560‐nm fiber laser. The effects of cavity dispersion and the length of a periodically poled lithium niobate (PPLN) gain element on the output spectrum and pulse duration are investigated by taking advantage of a very broad (∼ 1000 cm⁻¹) gain bandwidth near the 3.1‐μm OPO degeneracy point. A new method of assessing the total OPO group delay dispersion across its entire spectrum is proposed, based on measuring spectral signatures of trace amounts of molecular gases injected into the OPO cavity.     

All-solid-state cw mode-locked picosecond KTiOAsO4 (KTA) optical parametric oscillator

4 (KTA) optical parametric oscillator (OPO) synchronously pumped by a cw diode-pumped mode-locked Nd:YVO₄ oscillator–amplifier system. The laser system (pumped by 84 W of cw 808-nm diode radiation) generates 7-ps-long pulses at 1.064 μm with a repetition rate of 83.4 MHz and an average power of 29 W. The OPO, synchronously pumped by the 1.064-μm laser pulses, consists of a 15-mm-long KTA crystal (cut for type II noncritical phase-matching) in a folded signal resonant linear resonator. The average powers of the 1.54-μm signal radiation and the 3.47-μm idler radiation are 14.6 W and 6.4 W, respectively. The total OPO output of 21 W corresponds to an internal efficiency of 75%. The experimental investigations include measurements of the OPO output power (and its dependence on the pump power, the transmission of the output coupler, and the resonator length) and of the pulse properties (such as pulse duration and spectral width). The measured results are in good agreement with the predictions of a numerical analysis based on a split-step Fourier method. Received: 4 May 1998     

Efficient, single frequency, high repetition rate, PPLN OPO pumped by a prelase Q-switched diode-pumped Nd:YAG laser

Single axial mode operation (<200 MHz optical bandwidth) of a high repetition rate periodically poled lithium niobate optical parametric oscillator (OPO) has been obtained at signal wavelengths between 1.46 μm and 1.64 μm. OPO signal slope efficiencies of 35% have been measured for repetition rates of 5–20 kHz. Single mode operation required spectral narrowing of both the pump laser and the OPO. A simple technique of prelase Q-switching was implemented to reduce the optical bandwidth of the cw diode-pumped Nd:YAG pump laser to <1 GHz. A single intracavity étalon was then sufficient to ensure single frequency oscillation of the OPO signal. The OPO output was stable with a smooth spatial profile and an M ² value of 1.3. Received: 29 September 1999 / Published online: 27 January 2000     

The line shape of Coherent Antistokes Raman Scattering excited under resonance conditions

If Coherent Antistokes Raman Scattering (CARS) is excited by a biharmonic pump in resonance with one photon molecular transition, the Raman line shape is significantly changed compared to the case of nonresonant excitation. This is due to an essential alteration of the 3rd order susceptibility responsible for CARS. Aside from a considerable enhancement of the Raman signal (which makes it possible to detect dissolved molecules in concentration lower than 10^-4 mole/l) the most marked effects are a reversal of the spectral sequence of the Raman maximum and minimum in the CARS-spectrum and for a specially chosen concentration an enhanced contrast of the signal. The factors determining the CARS line shape are discussed theoretically and preliminary experimental results concerning resonance CARS spectra are reported.     

Wavelength-swept optical parametric oscillator for broadband mid-infrared spectroscopy

In this work we describe a wavelength-swept continuous-wave optical parametric oscillator (OPO) for the rapid acquisition of mid-infrared spectra spanning over hundreds of wavenumbers. Rapid tuning of a ytterbium-doped fibre pump laser resulted in the OPO idler tuning over 900 cm^?1 in 3.36 ms at a resolution of 4.5 cm^?1, within a total accessible range of 2.67 to 4.34 μm (2304–3752 cm^?1). Predictable tuning characteristics allowed simple online calibration of recorded spectra for absolute mid-infrared frequency. The system thus offers a viable approach to broadband spectral acquisition in applications requiring high-radiance illumination.     

Broadly tunable dual-wavelength light source for coherent anti-Stokes Raman scattering microscopy

The signal and idler beams from a picosecond, synchronously pumped optical parametric oscillator (OPO) provide the two colors necessary for coherent anti-Stokes Raman scattering (CARS) microscopy. The OPO provides a continuously tunable frequency difference between the two beams over a broad range of Raman shifts (100-3700 cm(-1)) by varying the temperature of a single nonlinear crystal. The near-infrared output (900-1300 nm) allows for deep penetration into thick samples and reduced nonlinear photodamage. Applications of this light source to in vivo cell and ex vivo tissue imaging are demonstrated.     

Spectral properties and numerical modelling of a critically phase-matched nanosecond LiB3O5 Optical Parametric Oscillator

We report on the spectral properties and numerical modelling of a singly resonant Optical Parametric Oscillator (OPO) of Lithium-triBOrate (LBO). The OPO is pumped by the second, third or fourth harmonic of an injection-seeded,Q-switched Nd:YAG laser. The measured OPO parameters are the tuning range, the threshold, the spectral linewidth, the efficiency and the output power. For the LBO-OPO critical type-I phase-matching (xy-plane) provides a wide tuning range and optimum values of the effective nonlinear coefficient. Pumped, for example, by the 355nm third harmonic the spectral range extends from 414 nm to 2.47 µm. With a 15 mm long crystal the OPO generates a total output-pulse energy of 77 mJ with an efficiency of 45%. For 266 nm pump radiation the signal wave is in the near ultraviolet at 307–325 nm. If pumped at 532 nm the OPO generates simultaneously two pairs of signal and idler waves in the infrared (0.707–2.15 µm). The pulse-energy fluctuations of the two pairs are correlated. If, however, the OPO is injection-seeded at one of the signal waves the two wavelength pairs are anti-correlated. The observed wavelength tuning as well as the measured spectral line-widths, threshold energies and efficiencies are in agreement with the values predicted by computer simulation.     

Femtosecond OPO based on LBO pumped by a frequency-doubled Yb-fiber laser-amplifier system for CARS spectroscopy

We present detailed investigations of a femtosecond green-pumped optical parametric oscillator (OPO) based on lithium triborate. As pump source, a frequency-doubled Yb-fiber laser-amplifier system is used. The OPO generates signal pulses tunable over a spectral range from 780 to 940 nm and idler pulses tunable from 1630 to 1190 nm. More than 250 mW are generated in the signal beam and more than 300 mW in the idler beam. Without dispersion compensation chirped signal pulses with a pulse duration between 100 and 250 fs are measured. Using this system for coherent anti-Stokes Raman scattering spectroscopy, vibrational resonances between 1110 and 6760 cm⁻¹ can be excited. Due to the chirped pulses, a spectral resolution of 100 cm⁻¹ is achieved, which is 2.5 times higher compared to an excitation with time-bandwidth limited pulses.     

Cylindrical KTiOPO(4) crystal for enhanced angular tunability of phase-matched optical parametric oscillators

We demonstrate what is to our knowledge the first optical parametric oscillator (OPO) based on a cylindrical phase-matched nonlinear crystal. Experiments are performed with an uncoated KTiOPO(4) crystal cut as a cylinder with a diameter of 21.2 mm and a thickness of 5 mm. We achieve 20.5-internal-degree angular tuning in a 1064-nm-pumped type III OPO, which would correspond to ~36 degrees external angular rotation for a parallelepipedal crystal. The goals of using such a device are to narrow the spectral width of the output and to enhance the spatial quality of the generated beams in widely tunable OPO's.