High-contrast coercive field engineering for periodic poling of RbKTiOPO4 with Ba2+/K+ ion-exchange.

We investigate a new method of coercive field engineering for periodic poling of RbKTiOPO4 (RKTP). By ion exchanging RKTP in a molten salt containing 7 mol% Ba(NO3)2 and 93 mol% KNO3 we achieve more than an order of magnitude difference in polarization switching time between the exchanged and non-exchanged regions. This method is used to fabricate periodic gratings of 2.92 µm in 1 mm thick bulk RKTP for second harmonic generation at 779 nm with a normalized conversion efficiency of 2%/Wcm. We show that the poled domain structures are stable at 300 °C, and that there is no bulk refractive index modification associated with the periodic ion exchange.

Multi-transversal mode pumping of narrow-bandwidth backward wave optical parametric oscillator.

A stable, narrow-bandwidth (274 MHz) backward wave optical parametric oscillator (BWOPO) generating mJ-level backward signal at 1885nm and forward idler at 2495 nm is presented. The BWOPO was pumped by a single-longitudinal mode, Q-switched Nd:YAG high-energy laser at 1064 nm. We show that multi-transversal mode pumping leads to the spectral broadening of the BWOPO backward signal and the generation of nanosecond pulses 2.7 times above the Fourier transform limit. We demonstrate over 100 GHz continuous tuning of the parametric output by adjusting the temperature of the BWOPO crystal, showcasing the significant role of thermal expansion in tuning performance. The BWOPO signal was used as a seed for a single-stage PPRKTP optical parametric amplifier (OPA) to boost the narrowband signal and idler energies to 20 mJ. This combination of mJ-level BWOPO seed with a single-stage PPRKTP OPA comprises a simple concept that would benefit long-range differential absorption lidar (DIAL) in the near and mid-infrared regions.

Highly efficient, high average power, narrowband, pump-tunable BWOPO.

We demonstrate a continuously tunable mid-infrared source that produces narrowband radiation at 1981 nm and 2145 nm based on a tunable Yb-based hybrid MOPA pump and a backward-wave optical parametric oscillator (BWOPO). The BWOPO employs a PPRKTP crystal with 580 nm domain periodicity. The BWOPO has a record-low oscillation threshold of 19.2 MW/cm2 and generates mJ level output with an overall efficiency exceeding 70%, reaching an average power of 5.65W at the repetition rate of 5 kHz. The system is mechanically robust and optical cavity-free, making it suitable for spectroscopic systems on mobile platforms. The mid-infrared signal frequency is tuned by pump tuning with a linear pump-to-signal frequency translation rate close to the predicted 1 to 1.001 Hz/Hz.

All-silica optical fiber bonding.

In this work, we demonstrate a spot-welding method for fabrication of all-silica fiber components. A CO2 laser was used to locally sinter sub-micron silica powders, enabling rigid bonding of optical fiber to glass substrates. The bonding was achieved without inducing any fiber transmission losses. The components showed no sign of deterioration or structural change when heated up to 1100 °C. These single material assemblies are therefore well suited for use in harsh environments where high stability and robustness is required.

Self-mode-locking through intra-cavity sum-frequency generation.

A new technique for mode-locking is demonstrated based on two lasers sharing one leg for sum-frequency generation. When the two lasers had equal round trip time one will produce bright pulses and the other dark pulses. Both lasers used Nd:YVO4 as the gain material, but operated at different wavelengths, namely 1064 nm and 1342 nm. In the present configuration, sub-250 ps pulses were generated at a repetition rate of 276 MHz with an output power of 70 mW. With appropriate choice of round trip loss at the two wavelengths it was possible to choose which laser was generating the bright pulses.

Optical poling by means of electrical corona discharge.

Electrical corona discharge is employed in this work to deposit ions on the surface of an optical fiber, creating a strong electric field that is used for poling. Green laser light propagating in the core frees photocarriers that are displaced by the poling field. The technique presented can induce a higher optical nonlinearity than previously obtained in traditional optical poling with internal metal electrodes. To date, a maximum second order nonlinearity 0.13 pm/V has been achieved for a 15 kV corona discharge bias.

Electrooptic control of the modal distribution in a silicate fiber.

We demonstrate the use of the electrooptic effect to control the propagation constant of the guided modes in silicate few mode fibers with internal electrodes. The electrooptic effect induces a perturbation of the fiber's refractive index profile that controls intermodal interference. To increase the electrooptic effect the silicate fibers are poled. The response time is in the nanosecond range.

Intra-cavity dark pulse generation through synchronized sum-frequency mixing.

A Nd:YVO4 laser operating at 1064 nm generating a stable mode-locked train of 10 ps-long dark pulses with a 211 MHz repetition rate is presented. The mode-locking relies on a periodic loss modulation produced by intra-cavity sum-frequency mixing with a synchronous bright-pulse train from a mode-locked femtosecond Yb:KYW laser at 1040 nm. A modulation depth of 90% was achieved for the dark pulses, confirmed by cross-correlation measurements. The ultrafast loss modulation injects power into the Nd:YVO4 laser cavity modes beyond the laser gain bandwidth. At proper laser cavity length, the detuning interaction of these modes with the lasing modes leads to the generation of periodic ultrafast transients at frequencies above 1.5 THz.

Room temperature photon-counting lidar at 3 µm.

A midinfrared single-photon-counting lidar at 3 µm is presented. The 3 µm photons were upconverted to 790 nm in a periodically poled rubidium-doped KTiOPO4 crystal through intracavity mixing inside a 1064 nm Nd:YVO4 laser and detected using a conventional silicon single-photon avalanche detector (SPAD). The lidar system could distinguish 1 mm deep features on a diffusely reflecting target, limited by the SPAD and time-tagging electronics. This technique could easily be extended to longer wavelengths within the transparency of the nonlinear crystal.

Intracavity interrogation of an array of fiber Bragg gratings.

In this work, we explore the interrogation of an array of fiber Bragg gratings as part of a laser cavity. A semiconductor optical amplifier in a sigma-shaped fiber cavity provides gain and is gated periodically at a rate that matches the roundtrip time associated with each grating of the array. The interrogator exhibits clear laser properties such as a threshold and linewidth narrowing. Besides improving the signal-to-noise ratio and enabling the re-use of wavelengths, it is found that this interrogation scheme enables monitoring of weak gratings spaced by less than 1 cm. Intracavity grating interrogation studied here is found to be a simple and powerful way to increase the number of sensor points for industrial applications.

Two-photon-absorption enhanced terahertz generation from KTP optically pumped in the visible-to-UV range.

By generating terahertz pulses in KTP crystals through optical rectification with a pump photon energy varying from below to above the bandgap, we observe a peak of the THz signal at the bandgap energy but also a second one around half the bandgap. This later one is attributed to a two-photon absorption enhanced nonlinearity, which is validated by the similarity of the two-photon absorption coefficient and THz peak amplitude data versus the pump photon energy. A careful analysis of the KTP sample absorption spectral dependence nearby the bandgap demonstrates that KTP is an indirect bandgap crystal, whose absorption below the bandgap involves emission of a phonon related to the symmetric Ti-O stretching vibration, i.e. the ν1 (A1g) mode.

Octave-spanning supercontinuum generation from off-axis Raman oscillation in a monolithic KTP crystal.

We report generation of a visible and near-infrared supercontinuum from a high-gain, ultra-broadband, and mirrorless Raman oscillator in a monolithic KTP crystal. The plane transverse to the pump axis resonates and traps off-axis Stokes waves and their frequency-upconverted components bouncing between two crystal surfaces via total internal reflection. The Raman gain is maximized with the Stokes polarization perpendicular to the plane of reflections. When pumped by a Q-switched Nd:YAG laser, the monolithic oscillator generates quasi-mode-locked Stokes pulses with octave-spanning spectral groups across the visible and near-infrared spectra between 540 and 1800 nm.

µJ-level multi-cycle terahertz generation in a periodically poled Rb:KTP crystal.

We demonstrate multi-cycle terahertz (MC-THz) generation in a 15.5 mm long periodically poled rubidium (Rb)-doped potassium titanyl phosphate (Rb:PPKTP) crystal with a poling period of 300 µm. By cryogenically cooling the crystal to 77 K, up to 0.72 µJ terahertz energy is obtained at a frequency of 0.5 THz with a 3 GHz bandwidth. A maximum internal optical-to-terahertz conversion efficiency of 0.16% is achieved, which is comparable with results achieved using periodically poled lithium niobate crystal. Neither photorefractive effects nor damage was observed with up to 900mJ/cm2, showing the great potential of Rb:PPKTP for multi-millijoule-level MC-THz generation.

UV-written grating couplers on thin-film lithium niobate ridge waveguides.

Grating couplers on thin-film lithium niobate ridge waveguides were designed and fabricated using UV-laser ablation. The calculated coupling efficiency with a sinusoidal grating can be as large as 53% in a 0.5 µm thin film. The maximum grating depth we fabricated was 130nm, limiting the coupling efficiency to a theoretical value of 18%. We fabricated grating couplers on adhered ridge waveguides of 20 µm thickness. Coupling light to waveguides on thin-film lithium niobate is still challenging, and here we present a fast, cheap and reliable fabrication alternative. It will benefit the on-chip testing of integrated components developed on this novel and promising material platform.

C-cavity fiber laser employing a chirped fiber Bragg grating for electrically gated wavelength tuning.

We present a novel C-cavity concept for tunable lasers. The laser is based on a semiconductor optical amplifier (SOA), serving both as a gain medium as well as a modulator, and a chirped fiber Bragg grating (C-FBG) which acts as the end mirrors on both cavity ends. Driving the SOA with a pulse pair with variable delay enables wavelength tuning by targeting different regions in the C-FBG with the circulating pulse. The cavity design allows for wide tuning while maintaining a constant repetition rate, we show a tuning range of 35 nm -limited by the C-FBG's reflection bandwidth. Time-multiplexed operation with four different wavelengths is also demonstrated. Furthermore, the laser performance and dynamics under different operating conditions are analyzed and discussed.

Ion-exchanged waveguides in periodically poled Rb-doped KTiOPO4 for efficient second harmonic generation.

An ion-exchange process has been developed for periodically poled Rb-doped KTiOPO4 (RKTP) which warrants high efficiency and low loss channel waveguides. The domain stability was investigated, and it was found that domain gratings with uncharged walls could stand the ion-exchange process without deterioration. 3.1 mW of blue second harmonic light was generated from 74 mW of radiation at 940.2 nm coupled into an 8 µm wide and 7 mm long waveguide, corresponding to a normalized conversion efficiency of 115%/Wcm2. Waveguides in PPRKTP open the possibility for stable operation at high optical powers, as well as generating entangled photons at low optical powers, and enable the investigation of novel nonlinear processes such as counter-propagating interactions in a waveguide format.

Development and experimental demonstration of negative first-order quasi-phase matching in a periodically poled Rb-doped KTiOPO4 crystal.

We worked on a new scheme of quasi-phase matching (QPM) based on the negative first order of the spatial modulation of the sign of the second-order nonlinearity. Applying this scheme in the case of angular-QPM (AQPM) in a biaxial crystal reveals new directions of propagation for efficient parametric frequency conversion as well as "giant" spectral acceptances. The experimental validation is performed in a periodically poled rubidium-doped KTiOPO4 biaxial crystal. This new approach naturally extends to other periodically poled uniaxial crystals such as periodically poled LiNbO3.

High resolution and sensitivity up-conversion mid-infrared photon-counting LIDAR.

A single-photon-counting mid-infrared LIDAR is presented. 2.4 µm mid-infrared photons were up-converted to 737 nm by intra-cavity mixing in a periodically poled rubidium-doped KTiOPO4 crystal inside a Nd:YVO4 laser. The up-converted photons were detected by a Si single-photon avalanche photodiode (SPAD). A temporal resolution of 42 ps and a dark count rate of 500 Hz were achieved, limited by the SPAD and ambient light leakage. It allowed for detection of two targets separated by only a few millimeters. This technique is easily extendable to longer wavelengths, limited primarily by the nonlinear crystal transparency.

Narrowband, tunable, infrared radiation by parametric amplification of a chirped backward-wave OPO signal.

The strict momentum conservation constraints for backward-wave optical parametric oscillators (BWOPOs) gives an inherently narrowband backward-generated wave, even with broadband pumping. Unfortunately, the limited tuning range of this wave restricts potential applications. Here we demonstrate a method to circumvent this restriction and increase the tuning range by more than one order of magnitude. A linearly chirped pump modulation is transferred to the forward-generated BWOPO wave, which is then mixed with an identically chirped pump in a conventional optical parametric amplifier to obtain narrowband (38 GHz), broadly tunable, infrared radiation around 1.86 µm, with an output energy of 19 µJ.

1.57 µm fiber source for atmospheric CO2 continuous-wave differential absorption lidar.

We present an efficient fiber source designed for continuous-wave differential absorption light detection and ranging (CW DIAL) of atmospheric CO2-concentration. It has a linewidth of 3 MHz, a tuning range of 2 nm over the CO2 absorption peaks at 1.572 µm, and an output power of 1.3 W limited by available pump power. Results from the initial CW DIAL testing are also presented and discussed.