Optical Frequency Doubling Research Articles

On the generation of optical third-harmonic through quasi-phase-matching in quadratic nonlinear crystals

We investigate the simultaneous processes of optical frequency doubling and sum frequency generation in quasi-phase-matched quadratically nonlinear crystals. Specifically, we focus on lattices with domain thicknesses corresponding to the coherence lengths of the two cascaded parametric processes and explore the realistic scenario of a single crystal containing sequential domain segments, each independently satisfying quasi-phase-matching for optical frequency doubling and summing, respectively. We demonstrate that high conversion efficiencies to the third harmonic can be obtained from an arbitrary fundamental wavelength.

Robust single-sideband-modulated Raman light generation for atom interferometry by FBG-based optical rectangular filtration.

Low-phase-noise and pure-spectrum Raman light is vital for high-precision atom interferometry by two-photon Raman transition. A preferred and prevalent solution for Raman light generation is electro-optic phase modulation. However, phase modulation inherently brings in double sidebands, resulting in residual sideband effects of multiple laser pairs beside Raman light in atom interferometry. Based on a well-designed rectangular fiber Bragg grating and a plain electro-optic modulator, optical single-sideband modulation has been realized at 1560 nm with a stable suppression ratio better than -25 dB despite of intense temperature variations. After optical filtration and frequency doubling, a robust phase-coherent Raman light at 780 nm is generated with a stable SNR of better than -19 dB and facilitates measuring the local gravity successfully. This FBG-based all-fiber single-sideband-modulated Raman light source, proposed for the first time and characterized as robust, compact and low-priced, is practical and potential for field applications of portable atom interferometry.

Broadband ultrafast-laser-absorption imaging in the ultraviolet for spatiotemporally resolved measurements of temperature and CN

This Letter describes the development and implementation of an ultraviolet and broadband ultrafast-laser-absorption-imaging (UV-ULAI) diagnostic for one dimensional (1D) imaging of temperature and CN via its B2Σ+←X2Σ+ absorption bands near 385 nm. The diagnostic was demonstrated by acquiring single-shot measurements of 1D temperature and CN profiles in HMX flames at a repetition rate of 25 Hz. Ultrashort pulses (55 fs) at 800 nm were generated using a Ti:Sapphire oscillator and then amplification and wavelength conversion to the ultraviolet were carried out using an optical parametric amplifier and frequency doubling crystals. The broadband pulses were spectrally resolved using a 1200 l/mm grating and imaged on an EMCCD camera to obtain CN absorbance spectra with a resolution of ≈0.065 nm and a bandwidth of ≈4 nm (i.e., 260 cm–1). Simulated absorbance spectra of CN were fit to the measured absorbance spectra using non-linear curve fitting to determine the gas properties. The spatial evolution of gas temperature and CN concentration near the burning surface of an HMX flame was measured with a spatial resolution of ≈10 μm. 1D profiles of temperature and CN concentration were obtained with a 1-σ spatial precision of 49.3 K and 4 ppm. This work demonstrates the ability of UV-ULAI to acquire high-precision, spatially resolved absorption measurements with unprecedented temporal and spatial resolution. Furthermore, this work lays the foundation for ultraviolet imaging of numerous atomic and molecular species with ultrafast time resolution.

Effect of erbium-doped fiber amplifier loss compensation on 5G new radio millimeter-wave seamless transmission over analog fiber and free space optical fronthaul at 60 GHz

We demonstrate a fifth generation (5G) new radio (NR) signal transmitted by an analog optical and seamless antenna wireless connection at the frequency band of 60 GHz to exploit a high-frequency unlicensed frequency range. An optical frequency doubling technique, using two Mach–Zehnder modulators operating in a carrier suppressed and linear regime, respectively, was adopted to obtain the desired millimeter wave frequency at the photodetector’s output. The proposed system was tested with the 5G NR signals with a maximum bandwidth of 200 MHz and 64 quadrature amplitude modulation format. It was shown that the signal transmitted through the optical fiber and free space optical link with 1 m long seamless antenna transmission at 62 GHz was capable of meeting the signal quality requirements in terms of error vector magnitude. Moreover, the system phase noise performance showed an almost negligible difference between the various system configurations.

Enhancement of terahertz wave emission from air plasma excited by harmonic three-color laser fields

Here we use the two-dimensional photocurrent model to analyze the terahertz (THz) wave generation from the air plasma excited by the harmonic three-color femtosecond lasers with wavelengths of 1200 nm, 600 nm, and 400 nm, respectively. This three-color harmonic frequency combination is readily-accessible under laboratory condition by using the suitable optical parametric amplifier and frequency doubling crystals, without the need of complex optical components for third harmonic wave. Through theoretical simulations, we find that the addition of 400 nm ultrafast laser with extremely low energy to the two-color laser fields of 1200 nm and 600 nm could effectively enhance the THz output. The THz wave energy emitted from the plasma point source could be easily enhanced up to 5.8 times. By either replacing the 400 nm laser with 800 nm laser or changing the near-infrared fundamental wavelength from 1200 nm to 1600 nm, the enhancement effect all noticeably reduces. These results reveal that the three-color laser fields which satisfy the harmonic frequency relation could achieve the utmost THz yield compared with the common two-color fields or non-harmonic three-color fields. This is of great significance for obtaining more powerful THz sources. The optimal conditions for the strongest THz wave generation are also calculated as a guidance for application.

Influence of self- and cross-phase modulations on an optical frequency doubling process for metamaterials.

In the process of doubling the frequency of high-power laser radiation in metamaterials, the self- and cross-phase modulations associated with the cubic nonlinearity of the medium are studied in the constant-intensity approximation. It is assumed that the pump wave propagates in a region with negative refraction. It is shown that in the case of a nonlinear-optical process of frequency doubling of an intense light wave in a metamaterial, it is important to take into account the self-action and cross-interaction effects that directly affect the optimal phase relationship between the interacting waves. It is discovered that, as a result of counter interaction of waves, the period of spatial beats changes. According to the analytic expressions obtained in the work, the choice of optimal parameters of the problem makes it possible to realize the regime of effective frequency doubling. By varying the intensity of the pump, it is possible to control and manipulate the intensity of the output coherent radiation from the minimum to the maximum value. A numerical evaluation of the efficiency of the doubling process in a metamaterial is given, and a comparison is made with similar results in LiNbO3 and KDP crystals. It is demonstrated that the effects of self- and cross-modulation in media with negative refraction can be used as a powerful tool for developing effective frequency doublers in such materials.

A picosecond laser at 1 kHz with dual-length of regenerative amplifier for the SLR in the daytime

An average power of 3 W green picosecond laser at a pulse repetition frequency of 1 kHz was reported. At the Shanghai satellite laser ranging (SLR) station, many satellites were measured with this laser and a narrow spectrum filter in the daytime. Satellites of the Beidou satellite navigation system for the Chinese, Global navigation satellite system for the Russian and Galileo satellite navigation system for the Europe were measured respectively. Especially ranging precision root mean square was up to 19.4 mm for the satellite of compassi5 about 37,000 km far from the earth. A Nd:YVO4 mode-locked laser with an etalon, a dual-length of Nd:YAG regenerative amplification cavity, single-pass two Nd:YAG modules amplifier and nonlinear optical materials LiB3O5 frequency doubling were designed. An average power of 3 W in 532 nm was obtained with pulse width of 30 ps, and angle of divergence was 0.8 mrad. Using the dual-length of regenerative cavity would greatly decrease the laser's volume and improve stability of picosecond laser. A narrow spectrum filter can reduce a lot of optics noise from the sun, and so all the satellites were almost be measured with this laser in the daytime at the Shanghai SLR station.

Optomechanical Enhancement of Doubly Resonant 2D Optical Nonlinearity.

Emerging two-dimensional semiconductor materials possess a giant second order nonlinear response due to excitonic effects while the monolayer thickness of such active materials limits their use in practical nonlinear devices. Here, we report 3300 times optomechanical enhancement of second harmonic generation from a MoS2 monolayer in a doubly resonant on-chip optical cavity. We achieve this by engineering the nonlinear light-matter interaction in a microelectro-mechanical system enabled optical frequency doubling device based on an electrostatically tunable Fabry-Perot microresonator. Our versatile optomechanical approach will pave the way for next generation efficient on-chip tunable light sources, sensors, and systems based on molecularly thin materials.

Tunable cw UV laser with

We present a solid-state laser system that generates over 200 mW of continuous-wave, narrowband light, tunable from 316.3 nm - 317.7 nm and 318.0 nm - 319.3 nm. The laser is based on commercially available fiber amplifiers and optical frequency doubling technology, along with sum frequency generation in a periodically poled stoichiometric lithium tantalate crystal. The laser frequency is stabilized to an atomic-referenced high finesse optical transfer cavity. Using a GPS-referenced optical frequency comb we measure a long term frequency instability of < 35 kHz for timescales between 10(-3) s and 10(3) s. As an application we perform spectroscopy of Sr Rydberg states from n = 37 - 81, demonstrating mode-hop-free scans of 24 GHz. In a cold atomic sample we measure Doppler-limited linewidths of 350 kHz.

二次谐波产生的光学倍频腔自锁定系统

二次谐波产生的光学倍频腔自锁定系统

Er-fiber femtosecond optical frequency comb covering visible light

Femtosecond optical frequency combs (FOFCs) with output wavelengths covering visible light have potential applications in absolute frequency measureflent of iodine-stabilized lasers and optical clock lasers. Based on optical amplification, frequency doubling and spectrum broadening, a home-made Er-fiber femtosecond optical frequency comb (Er-FOFC) with output wavelengths covering visible light is demonstrated. One path with an average power of 8 mW from Er-FOFC is used as the seed pulse for spectrum broadening to cover the visible light. This path is first amplified to 532 mW by injecting into an Er-doped femtosecond fiber amplifier with combined forward and backward pumping and then frequency doubled with a MgO: PPLN crystal with an output power of 85 mW, frequency-doubling efficiency of 32% and pulse duration of 85fs. The output power of this path can be first amplified to 532 mW through an Er-doped femtosecond fiber amplifier when the forward pumping and backward pumping both turn on. Then the frequency-doubling laser can be generated in a MgO: PPLN crystal. The frequency-doubling efficiency is 32% and the pulse duration is 85 fs; the frequency-doubling light is spectrally broadened from 500 to 1000 nm in a photonic crystal fiber (PCF), with an output power of 85 mW and coupling efficiency of 50%. To verify the performance of the broadened spectrum, the light from the Er-FOFC and a compact iodine-stabilized frequency-doubled Nd: YAG laser at 532 nm is beaten. A beat signal with a signal-to-noise ratio of 30 dB at 100 kHz RBW is obtained, which provides a useful tool for absolute frequency measureflent of visible lasers.

Depth-encoded all-fiber swept source polarization sensitive OCT.

Polarization sensitive optical coherence tomography (PS-OCT) is a functional extension of conventional OCT and can assess depth-resolved tissue birefringence in addition to intensity. Most existing PS-OCT systems are relatively complex and their clinical translation remains difficult. We present a simple and robust all-fiber PS-OCT system based on swept source technology and polarization depth-encoding. Polarization multiplexing was achieved using a polarization maintaining fiber. Polarization sensitive signals were detected using fiber based polarization beam splitters and polarization controllers were used to remove the polarization ambiguity. A simplified post-processing algorithm was proposed for speckle noise reduction relaxing the demand for phase stability. We demonstrated systems design for both ophthalmic and catheter-based PS-OCT. For ophthalmic imaging, we used an optical clock frequency doubling method to extend the imaging range of a commercially available short cavity light source to improve polarization depth-encoding. For catheter based imaging, we demonstrated 200 kHz PS-OCT imaging using a MEMS-tunable vertical cavity surface emitting laser (VCSEL) and a high speed micromotor imaging catheter. The system was demonstrated in human retina, finger and lip imaging, as well as ex vivo swine esophagus and cardiovascular imaging. The all-fiber PS-OCT is easier to implement and maintain compared to previous PS-OCT systems and can be more easily translated to clinical applications due to its robust design.

A laser setup for rubidium cooling dedicated to space applications

We present the complete characterization of a laser setup for rubidium cooling dedicated to space applications. The experimental setup is realized with commercial off-the-shelf fiber components suitable for space applications. By frequency doubling two fiber laser diodes at 1560 nm, we produce the two optical frequencies at 780 nm required for atomic cooling of 87Rb. The first laser is locked on saturated absorption signal and long-term frequency drift has been canceled using a digital integrator. The optical frequency of the second laser is controlled relatively to the first one by a frequency comparison method. A full characterization of the setup, including frequency stability evaluation and frequency noise measurement, has been performed. The optical frequency doubling module has been submitted to environmental tests to verify its compatibility with space applications.

Optical frequency tripling with improved suppression and sideband selection

A novel optical dispersion tolerant millimetre-wave radio-over-fibre system using optical frequency tripling technique with enhanced and selectable sideband suppression is demonstrated. The implementation utilises cascaded optical modulators to achieve either an optical single sideband (OSSB) or double sideband-suppressed carrier (DSB-SC) signal with high sideband suppression. Our analysis and simulation results indicate that the achievable suppression ratio of this configuration is only limited by other system factors such as optical noise and drifting of the operational conditions. The OSSB transmission system performance is assessed experimentally by the transport of 4 WiMax channels modulating a 10 GHz optical upconverted RF carrier as well as for optical frequency doubling and tripling. The 10 GHz and tripled carrier at 30 GHz are dispersion tolerant resulting both in an average relative constellation error (RCE) of -28.7 dB after 40 km of fibre.

Novel 60 GHz RoF system with optical single sideband mm-wave signal generation and wavelength reuse for uplink connection

In order to improve RF frequency to achieve higher bandwidth and larger capacity, we propose a novel scheme to generate optical single sideband (SSB) millimeter-wave, in which frequency doubling of local radio frequency (RF) is obtained by using one integrated Mach–Zehnder modulator (MZM), and we theoretically investigate the generating principle of SSB. The optical SSB modulation scheme is employed to generate 60 GHz optical mm-wave and the 2.5 Gb/s baseband signal is simultaneously up-converted at the central station (CS) for downlink transmission, and the optical carrier is reused for uplink connection at the base station (BS). The full-duplex 2.5 Gb/s data are successfully transmitted over 40 km standard single-mode fiber (SMF-28) for both uplink connection and downlink connection with less than 2-dB power penalty. Results show the novel 60 GHz RoF system with optical SSB mm-wave signal generation using optical frequency doubling is feasible and we can obtain simple cost-efficient configuration and good performance over long-distance transmission.

Naturally Phase-Matched Second-Harmonic Generation in a Whispering-Gallery-Mode Resonator

We demonstrate for the first time natural phase matching for optical frequency doubling in a high-Q whispering-gallery-mode resonator made of lithium niobate. A conversion efficiency of 9% is achieved at 30 microW in-coupled continuous wave pump power. The observed saturation pump power of 3.2 mW is almost 2 orders of magnitude lower than the state-of-the-art value. This suggests an application of our frequency doubler as a source of nonclassical light requiring only a low-power pump, which easily can be quantum noise limited. Our theoretical analysis of the three-wave mixing in a whispering-gallery-mode resonator provides the relative conversion efficiencies for frequency doubling in various modes.

Optical frequency doubling for multichannel radio-over-fibre system based on integrated phase modulator and silicon coupled microring notch filter

An integrated optical millimetre-wave signal generator based on a coupled microring resonator, which doubles the frequency of the electrical signal driving a phase modulator, is proposed and experimentally demonstrated for future radio-over-fibre systems. The coupled microring notch filter was fabricated on a silicon-on-insulator wafer. A high extinction ratio optical millimetre-wave signal was obtained.

Optical rectification and frequency doubling in metal vapors

It is theoretically shown that, under conditions of two-photon absorption of radiation by metal vapors, the medium becomes considerably polarized and efficiently doubles the radiation frequency. Two physical mechanisms by which these effects can occur are detected. The first of them results from the ring frequency mixing, while the second one is caused by a polarization nonlinearity that is quadratic in the field amplitude and that is formed as a result of the electric quadrupole absorption due to the forbidden transition. The effects predicted can form the basis for new optical methods for measuring the electric quadrupole and magnetic dipole moments for forbidden atomic transitions.

First order optical nonlinearities [formula omitted] for organic nanofibers from functionalized para-phenylenes

Optical frequency doubling is demonstrated from nanoscaled, parallely oriented needle-like organic aggregates (‘nanofibers’) that have been grown on mica surfaces from unsymmetrically functionalized para-quaterphenylene organic molecules. Wavelength dependent absolute values of the first order non-linear susceptibility, χ zzz ( 2 ) , are measured for nanofibers from six differently functionalized molecules. These quantitative measurements are enabled by combining relative second-harmonic intensity measurements with reference to a quartz crystal with morphological information on the individual nanoaggregates gathered from atomic force microscopy images. Values up to that of KDP ( d 36 ) are found for nanofibers from specific functionalized molecules.

Phase Matching Characteristics of Second-Harmonic Generation in Periodic 90°-Domain Structures of KNbO3

Phase matching characteristics of second-harmonic generation (SHG) in periodic 90°-domain structures of KNbO3 single crystal were extensively analyzed. Important parameters including types of interaction and the corresponding effective nonlinear optical coefficients, grating periods for quasi-phase matching (QPM), spectral bandwidths, and group velocity mismatches were estimated. The results show that KNbO3 with a specific internal structure of periodic 90°-domains can be used for efficient narrowband second-harmonic generation and group-velocity-matched broadband optical frequency doubling near the optical communication window.