David J. M. Stothard

Low-pump-threshold continuous-wave singly resonant optical parametric oscillator

We describe a compact all-solid-state continuous-wave singly resonant optical parametric oscillator (SRO) with a minimal pump-power requirement. The SRO is based on periodically poled LiNbO(3) as the nonlinear material and is pumped by a 1-W diode-pumped Nd:YVO(4) minilaser at 1.064 microm . By exploiting the intracavity pumping technique in a 50-mm crystal, we have achieved SRO operation threshold at a diode pump power of only 310 mW.At 1 W of input diode power, the SRO delivers 70 mW of output power in the nonresonant idler at 3.66 microm , at a photon conversion efficiency of 55%. Multiparameter tuning of the SRO yields a signal wavelength range from 1.45 to 1.60 microm and an idler wavelength range from 3.16 to 4.02 microm in the mid infrared. The device is characterized by robust turnkey operation and long-term amplitude-stable performance.

Stable, continuous-wave, intracavity, optical parametric oscillator pumped by a semiconductor disk laser (VECSEL).

We report relaxation oscillation free, true continuous-wave operation of a singly-resonant, intracavity optical parametric oscillator (OPO) based upon periodically-poled, MgO-doped LiNbO3 and pumped internal to the cavity of a compact, optically-excited semiconductor disk laser (or VECSEL). The very short upper-laser-state lifetime of this laser gain medium, coupled with the enhancing effect of the high-finesse pump laser cavity in which the OPO is located, enables a low threshold, high efficiency intracavity device to be operated free of relaxation oscillations in continuous-wave mode. By optimizing for low-power operation, parametric threshold was achieved at a diode-laser power of only 1.4 W. At 8.5 W of diode-laser power, 205 mW of idler power was extracted, indicating a total down-converted power of 1.25 W, and hence a down-conversion efficiency of 83%.

Mode-hop-free tuning over 80 GHz of an extended cavity diode laser without antireflection coating

We report 80 GHz of mode-hop-free tuning from an extended cavity diode laser (ECDL) without the need for antireflection coating on the diode facet. A large continuous tuning range was achieved by using a simple electronic circuit which simultaneously varies the diode laser current and external cavity length in an appropriate ratio. The ECDL employs a commercial, single mode AlGaAs diode laser, operating at 814 nm, in a Littrow external cavity configuration. The applicability of this technique to the most commonly available single-mode diode lasers without the requirement of antireflection coating, makes it an attractive approach for the attainment of mode-hop-free tuning over extended ranges.

Mid-infrared photonic crystal waveguides in silicon

We demonstrate the design, fabrication and characterization of mid-infrared photonic crystal waveguides on a silicon-on-insulator platform, showing guided modes in the wavelength regime between 2.9 and 3.9 µm. The characterization is performed with a proprietary intra-cavity Optical Parametric Oscillator in a free space optical setup and with a fibre coupled setup using a commercial Quantum Cascade Laser. We discuss the use of an integrated Mach-Zehnder interferometer for dispersion measurements and report a measured group velocity of up to a value of n(g) = 12, and determine the propagation loss to be 20 dB/cm.

Hyperspectral imaging of gases with a continuous-wave pump-enhanced optical parametric oscillator

We demonstrate a system for the active real-time hyperspectral imaging of gases using a combination of a compact, pump-enhanced, continuous-wave optical parametric oscillator as an all-solid-state mid-infrared source of coherent radiation and an electro-mechanical polygonal imager. The wide spectral coverage and high spectral resolution characteristics of this source means that the system is capable of being selectively tuned into the absorption features of a wide variety of gaseous species. As an example we show how the largest absorption coefficient exhibited by methane at 3057.7cm(-1) can be accessed (amongst others) and gas plumes imaged in concentrations as low as 30ppm.m using a parametric oscillator based on periodically-poled RbTiOAsO(4) (PP-RTA).

Line-narrowed, compact, and coherent source of widely tunable terahertz radiation

Parametric down-conversion techniques provide an effective means for producing terahertz radiation. A novel device, based on an intra-cavity geometry, has been developed that is both compact and widely tunable over the range 1-3 THz. Reduction of the free-running linewidth of this device (typically 50-100 GHz) to <5 GHz accompanied by continuous and smooth tuning over a band extending over 50-100 GHz where this band may be set anywhere within the above tuning range is described. A significant increase in the mean terahertz power generated to >10 muW is attained by increasing the pulse repetition rate up to 350 Hz. Applications in spectroscopy and imaging are discussed.

Continuous-wave pump-enhanced optical parametric oscillator with ring resonator for wide and continuous tuning of single-frequency radiation

We demonstrate a PPLN based pump-enhanced, singly-resonant optical parametric oscillator configured in a traveling wave geometry and pumped by a Ti:sapphire laser. The inclusion of a low finesse etalon within the OPO cavity stabilizes the signal frequency, and rotation of the etalon allows this frequency to be systematically hopped from axial mode to nearest neighbor axial mode over the entire free spectral range of the etalon (83GHz). Tuning of the pump frequency allows the signal frequency to be smoothly tuned over a cavity free spectral range. More than 35mW of single frequency idler power was generated in the spectral range 2800-3000nm for 600mW pump power. The superiority of traveling wave over standing wave geometries in these regards is discussed.

Transient dynamics of CW intracavity singly resonant optical parametric oscillators

We study the transient dynamics of CW intracavity, singly-resonant optical parametric oscillators (ICSROs). Relaxation oscillations of the 1064-nm pump light are measured, in both the presence and absence of parametric conversion, in a periodically poled LiNbO/sub 3/ (PPLN) CW ICSRO pumped intracavity to a Nd:YVO/sub 4/ laser. The presence of parametric conversion is shown to significantly increase both the damping times and oscillation frequencies of the dynamics; at three times the parametric oscillator threshold, these take values of 200 /spl mu/s and 2 MHz, respectively. Oscillation frequencies are shown to be in good agreement with theoretical values. The influence of energy diffusion and thermal lensing are analyzed to account for discrepences between measured and theoretical damping times. We discuss the implications of the dynamic behavior to the stable-power operation of CW ICSROs.

Relaxation oscillation suppression in continuous-wave intracavity optical parametric oscillators

We report a solution to the long standing problem of the occurrence of spontaneous and long-lived bursts of relaxation oscillations which occur when a continuous-wave optical parametric oscillator is operated within the cavity of the parent pump-laser. By placing a second nonlinear crystal within the pump-wave cavity for the purpose of second-harmonic-generation of the pump-wave the additional nonlinear loss thereby arising due to up-conversion effectively suppresses the relaxation oscillations with very little reduction in down-converted power.

An Intracavity Optical Parametric Oscillator With Very High Repetition Rate and Broad Tunability Based Upon Room Temperature Periodically Poled

We report a low-pump-threshold, very high-repetition-rate, singly resonant optical parametric oscillator (OPO) pumped internally to a diode-pumped, Q-switched Nd:YVO4 laser. The OPO, based upon a 27.0-32.5-mum periodically poled MgO:LiNbO3 crystal with fan grating design, exhibits a broad tunability range of 1.38-1.79 mum (Deltalambdas = 411 nm) and 2.62-4.64 mum (Deltalambdai = 2019 nm) in the signal and idler fields, respectively. For an external diode pump power of only 3 W, up to 150 mW of idler were extracted from the device or, when the signal field was output coupled, 330 and 100 mW of signal and idler, respectively, were simultaneously obtained. By taking advantage of the intracavity parametric cavity dumping process, repetition rates in excess of 350 kHz were achieved whilst maintaining high down-conversion efficiency. We demonstrate the potential of this device in spectroscopic applications with an absorption measurement of low concentration CH4.