Non-collinear generation of ultra-broadband parametric fluorescence photon pairs using chirped quasi-phase matching slab waveguides.

Many optical quantum applications rely on broadband frequency correlated photon pair sources. We previously reported a scheme for collinear emission of high-efficiency and ultra-broadband photon pairs using chirped quasi-phase matching (QPM) periodically poled stoichiometric lithium tantalate (PPSLT) ridge waveguides. However, collinearly emitted photon pairs cannot be directly adopted for applications that are based on two-photon interference, such as quantum optical coherence tomography (QOCT). In this work, we developed a chirped QPM device with a slab waveguide structure. This device was designed to produce spatially separable (photon pair non-collinear emission) parametric fluorescence photon pairs with an ultra-broadband bandwidth in an extremely efficient manner. Using a non-chirped QPM slab waveguide, we observed a photon pair spectrum with a full-width-at-half-maximum (FWHM) bandwidth of 26 nm. When using a 3% chirped QPM slab waveguide, the FWHM bandwidth of the spectrum increased to 190 nm, and the base-to-base width is 308 nm. We also confirmed a generation efficiency of 2.4×106 pairs/(µW·s) using the non-chirped device, and a efficiency of 8×105 pairs/(µW·s) using the 3% chirped device under non-collinear emission conditions after single-mode fiber coupling. This is, to the best of our knowledge, the first report of frequency correlated photon pairs generation using slab waveguide device as a source. In addition, using slab waveguides as photon pair sources, we performed two-photon interference experiments with the non-chirped device and obtained a Hong-Ou-Mandel (HOM) dip with a FWHM of 7.7 µm and visibility of 98%. When using the 3% chirped device as photon pair source, the HOM measurement gave a 2 µm FWHM dip and 74% visibility.

Influence of thermal stress on continuous-wave second-harmonic generation in periodically poled LiTaO3 crystals.

Thermal stress effects on continuous-wave second-harmonic generation in periodically poled LiTaO3 crystals are evaluated via a numerical simulation that is based on nonlinear propagation equations and a thermal conduction equation. The thermal performance and absorption coefficients used in the numerical simulation were determined by comparison with experimental results. The results show that the thermal stress caused by a small residual absorption would restrict the maximum output power of a second-harmonic-continuous-wave laser.

Efficient generation of ultra-broadband parametric fluorescence using chirped quasi-phase-matched waveguide devices.

We present a highly efficient photon pair source using chirped quasi-phase-matched (QPM) devices with a ridge waveguide structure. We developed QPM waveguide devices with chirp rates of 3% and 6.7%. Spectrum measurements reveal that the generated photons have bandwidths of 229 nm and 325 nm in full width at half maximum (FWHM), alternatively, 418 nm and 428 nm in base-to-base width for the 3% and 6.7% chirped devices, respectively, which are much broader than the bandwidth of 16 nm in FWHM observed with a non-chirp device. We also evaluate the generation efficiency of photon pairs from coincidence measurements using two superconducting single photon detectors (SSPDs). The estimated generation efficiencies of photon pairs were 2.7 × 106 pairs/s·µW and 1.2 × 106 pairs/s·µW for the 3% and 6.7% chirped devices, respectively, which are comparable to the generation efficiency for the non-chirp device of 2.7 × 106 pairs/s·µW. We also measured the frequency correlation of the photon pairs generated from the 6.7% chirped device. The experimental results clearly show the frequency correlation of the generated broadband photon pairs.

0.54 µm resolution two-photon interference with dispersion cancellation for quantum optical coherence tomography.

Quantum information technologies harness the intrinsic nature of quantum theory to beat the limitations of the classical methods for information processing and communication. Recently, the application of quantum features to metrology has attracted much attention. Quantum optical coherence tomography (QOCT), which utilizes two-photon interference between entangled photon pairs, is a promising approach to overcome the problem with optical coherence tomography (OCT): As the resolution of OCT becomes higher, degradation of the resolution due to dispersion within the medium becomes more critical. Here we report on the realization of 0.54 µm resolution two-photon interference, which surpasses the current record resolution 0.75 µm of low-coherence interference for OCT. In addition, the resolution for QOCT showed almost no change against the dispersion of a 1 mm thickness of water inserted in the optical path, whereas the resolution for OCT dramatically degrades. For this experiment, a highly-efficient chirped quasi-phase-matched lithium tantalate device was developed using a novel 'nano-electrode-poling' technique. The results presented here represent a breakthrough for the realization of quantum protocols, including QOCT, quantum clock synchronization, and more. Our work will open up possibilities for medical and biological applications.

Thermal characteristics of second harmonic generation by phase matched calorimetry.

We analyze a solution of the heat equation for second harmonic generation (SHG) with a focused Gaussian beam and simulate the temperature rise in SHG materials as a function of the second harmonic power and the focusing conditions. We also propose a quantitative value of the heat removal performance of SHG devices, referred to as the effective heat capacity Cα in phase matched calorimetry. We demonstrate the inverse relation between Cα and the focusing parameter ξ, and propose the universal quantity of the product of Cα and ξ for characterizing the thermal property of SHG devices. Finally, we discuss the strategy to manage thermal dephasing in SHG using the results from simulations.

Parasitic-light-suppressed quasi-phase-matched optical parametric oscillation device.

Nonlinear absorption - such as green-induced infrared absorption (GRIIRA) - increases the risk of the catastrophic damage during high peak- power wavelength conversion. We propose a novel concept to suppress parasitic green second-harmonic generation (SHG) in optical parametric oscillation (OPO) using specially engineered quasi-phase-matched (QPM) structures. This selective suppression was achieved by relative π-phase shift in only SHG not OPO. Compared with a periodic device, a parasitic-light-suppressed (PLS) QPM device produced smaller normalized conversion efficiency in green and maintained singly resonant OPO performance.

Noncollinear parametric fluorescence by chirped quasi-phase matching for monocycle temporal entanglement.

Quantum entanglement of two photons created by spontaneous parametric downconversion (SPDC) can be used to probe quantum optical phenomena during a single cycle of light. Harris [Opt. Express 98, 063602 (2007)] suggested using ultrabroad parametric fluorescenc generated from a quasi-phase-matched (QPM) device whose poling period is chirped. In the Harris's original proposal, it is assumed that the photons are collinearly generated and then spatially separated by frequency filtering Here, we alternatively propose using noncollinearly generated SPDC. In our numerical calculation, to achieve 1.2 cycle temporal correlation for a 532 nm pump laser, only 10% -chirped device is sufficien when noncollinear condition is applied, while a largely chirped (50%) device is required in collinear condition. We also experimentally demonstrate an octave-spanning (790-1610 nm) noncollinear parametric fluorescenc from a 10% chirped MgSLT crystal using both a superconducting nanowire single-photon detector and photomultiplier tube as photon detectors. The observed SPDC bandwidth is 194 THz, which is the largest width achieved to date for a chirped QPM device. From this experimental result, our numerical analysis predicts that the bi-photon can be compressed to 1.2 cycles with appropriate phase compensation.

1.34-µm Nd:YVO4 laser mode-locked by SHG-lens formation in periodically-poled stoichiometric lithium tantalate.

Self-starting, steady-state χ((2))-lens mode-locking of a 1.34-µm diode-pumped Nd:YVO(4) laser using intracavity second harmonic generation in PPMgSLT is demonstrated. Pulses as short as 3.6 ps with an average output power of ~1 W are obtained at a repetition rate of 120 MHz.

Thermal performance in high power SHG characterized by phase-matched calorimetry.

We proposed a method to determine device quality in heat removal. Temperature change depending on SH power was analyzed by fitting with a new model to characterize heat removal performance of SHG modules, named as phase-matched calorimetry (PMC). The thermal disposal performance of SHG devices was improved by combination of metal housing and reduced crystal aperture. With a tight aperture, we demonstrated a 19 W single-pass 532-nm SHG at a conversion efficiency of 26.5% in a 10-mm-long PPMgSLT crystal without saturation.

High-gain, wide-dynamic-range parametric interaction in Mg-doped LiNbO3 quasi-phase-matched adhered ridge waveguide.

With recent developments and optimizations for quasi-phase-matched adhered ridge waveguide (QPM-ARW), outstanding performances containing efficient amplification were demonstrated by difference frequency generation (DFG) and optical parametric amplification (OPA). A maximum channel conversion efficiency of +7.6 dB (570%) was achieved in a telecommunication band using a 50 mm-long device, when coupling with 160 mW pump. Simultaneously, the input signal was amplified up to +9.5 dB (890%).

High-power picosecond Nd:GdVO4 laser mode locked by SHG in periodically poled stoichiometric lithium tantalate.

Periodically poled stoichiometric lithium-tantalate is used for mode locking of a diode-pumped Nd:GdVO(4) laser by intracavity second-harmonic generation. Stable and self-starting operation is observed achieving average output powers of up to 5 W at a pulse-repetition rate of 107 MHz. The obtained pulse durations range from 6.5 ps at maximum output power down to 3.2 ps at 1.4 W.

Joint temporal density measurements for two-photon state characterization.

We demonstrate a technique for characterizing two-photon quantum states based on joint temporal correlation measurements using time-resolved single-photon detection by femtosecond up-conversion. We measure for the first time the joint temporal density of a two-photon entangled state, showing clearly the time anticorrelation of the coincident-frequency entangled photon pair generated by ultrafast spontaneous parametric down-conversion under extended phase-matching conditions. The new technique enables us to manipulate the frequency entanglement by varying the down-conversion pump bandwidth to produce a nearly unentangled two-photon state that is expected to yield a heralded single-photon state with a purity of 0.88. The time-domain correlation technique complements existing frequency-domain measurement methods for a more complete characterization of photonic entanglement.

Time-resolved single-photon detection by femtosecond upconversion.

We demonstrate a time-resolved single-photon detection technique based on ultrafast sum-frequency generation, providing femtosecond measurement capability for single photons in photonic quantum information processing. Noncollinear broadband upconversion in periodically poled MgO-doped stoichiometric lithium tantalate with an ultrafast pump and detection with a Si single-photon counter enable efficient detection of IR photons and temporal resolution of ~150 fs. We utilize the timing resolution to map the generation efficiency profile along the propagation axis of a periodically poled KTiOPO(4) crystal, revealing its local grating quality with millimeter resolution. We also apply the technique to two-photon coincidence measurements and directly demonstrate time anticorrelation between coincident-frequency entangled photons that are parametrically generated under extended phase-matching conditions.

Thermal effects in high-power CW second harmonic generation in Mg-doped stoichiometric lithium tantalate.

We investigated thermal behaviors of single-pass second-harmonic generation of continuous wave green radiation with high efficiency by quasi-phase matching in periodically poled Mg-doped stoichiometric lithium tantalate (PPMgSLT). Heat generation turned out to be directly related to the green light absorption in the material. Strong relation between an upper limit of the second harmonic power and confocal parameter was found. Single-pass second-harmonic generation of 16.1 W green power was achieved with 17.6% efficiency in Mg:SLT at room temperature.

320 Gbps to 10 GHz sub-clock recovery using a PPLN-based opto-electronic phase-locked loop.

We present successful extraction of a 10 GHz clock from single-wavelength 160 and 320 Gbps OTDM data streams, using an opto-electronic phase-locked loop based on three-wave mixing in periodically-poled lithium niobate as a phase comparator.

Broadly tunable ultraviolet light generation in a compact MgO-doped periodically-poled stoichiometric lithium tantalate optical parametric oscillator with a high-Q cavity.

A compact tunable UV laser source based on intracavity sum frequency generation in a MgO-doped periodically-poled stoichiometric lithium tantalate optical parametric oscillator is reported. UV output at an approximately 1 mW level of power over the range of 364 to 378 nm with a bandwidth of 0.5 nm was obtained with a crystal that has just one periodically-poled grating period. The full tuning range can be as much as approximately 68 nm, from 324 to 392 nm by varying the crystal temperature from room temperature to 250 degrees C. This will cover nearly the entire UVA range.

Bright narrowband source of photon pairs at optical telecommunication wavelengths using a type-II periodically poled lithium niobate waveguide.

We report on the generation of narrowband photon pairs at telecommunication wavelengths using a periodically poled lithium niobate waveguide that utilizes the nonlinear tensor element d(24) for type-II quasi phase matching. The FWHM bandwidth of the spontaneous parametric downconversion was 1 nm. The brightness of the photon pair source was ~6x10(5)/s/GHz when the pump power was 1 mW. The indistinguishability of the signal and idler photons generated by the degenerate spontaneous parametric downconversion process was studied in a Hong-Ou-Mandel type interference experiment.

Green-pumped high-power optical parametric oscillator based on periodically poled MgO-doped stoichiometric LiTaO3.

A high-power 532 nm-pumped multikilohertz nanosecond optical parametric oscillator using a periodically poled 1.0 mol.% MgO-doped stoichiometric lithium tantalate crystal that could be operated from room temperature to 200 degrees C without damage is reported. A broad continuous tuning range from 855 to 1410 nm was achieved within a single domain period. Efficient operation of high peak power and watt level average power with a power conversion of 62.5% was measured. These results show that a high-resolution high average power visible tunable source can be realized.

Powerful red-green-blue laser source pumped with a mode-locked thin disk laser.

We present a red-green-blue laser source with average powers of 8 W in the red, 23 W in the green, and 10.1 W in the blue. The entire pump power for the nonlinear conversion stages is provided by a single laser oscillator without any amplifier stages. Our system does not require any synchronized cavities, and all nonlinear crystals except one are critically phase matched at room temperature.

Application of periodically poled stoichiometric LiTaO3 for efficient optical parametric chirped pulse amplification at 1 kHz.

We demonstrate optical parametric chirped pulse amplification (OPCPA) at 1-kHz repetition rate in periodically poled stoichiometric LiTaO3 (PPSLT) with 1 mol % MgO doping. Diode pumping was used both for the fiber laser generating the femtosecond seed pulses and the nanosecond laser/amplifier employed as a pump source. The high gain ( approximately 63 dB) and large bandwidth (20 nm) obtained for single-stage nondegenerate OPCPA operation provide a compact and efficient solution for amplification of ultrashort pulses near 1.57 microm. Stretched femtosecond pulses could be amplified up to 39.5 microJ in a 7-mm long, 2-mm thick sample of PPSLT. The pulse duration of the amplified signal pulses (FWHM) after recompression amounted to 315 fs.