Intracavity Difference-frequency Mixing Research Articles

Realizing symmetry-guaranteed pairs of bound states in the continuum in metasurfaces

Bound states in the continuum (BICs) have received significant attention for their ability to enhance light-matter interactions across a wide range of systems, including lasers, sensors, and frequency mixers. However, many applications require degenerate or nearly degenerate high-quality factor (Q) modes, such as spontaneous parametric down conversion, non-linear four-wave mixing, and intra-cavity difference frequency mixing for terahertz generation. Previously, degenerate pairs of bound states in the continuum (BICs) have been created by fine-tuning the structure to engineer the degeneracy, yielding BICs that respond unpredictably to structure imperfections and material variations. Instead, using a group theoretic approach, we present a design paradigm based on six-fold rotational symmetry (C6) for creating degenerate pairs of symmetry-protected BICs, whose frequency splitting and Q-factors can be independently and predictably controlled, yielding a complete design phase space. Using a combination of resonator and lattice deformations in silicon metasurfaces, we experimentally demonstrate the ability to tune mode spacing from 2 nm to 110 nm while simultaneously controlling Q-factor.

Mid-infrared electronic wavelength tuning through intracavity difference-frequency mixing in Cr:ZnSe lasers

Mid-infrared tunable coherent light sources are used in various laser applications, such as trace gas detection, laser processing, and biomedical diagnostics. This study demonstrates mid-infrared generation in the 8.3–11 µm (i.e., 900–1200 cm−1) spectral range by configuring intracavity difference-frequency generation (DFG) using ZnGeP2 (ZGP) in an electronically tuned Cr:ZnSe laser. The broad tunability is achieved with the maximum pulse energies exceeding 100 μJ by combining the electronic wavelength tuning with sligh angle adjustments (Δθ < 0.5°) of ZGP under the spectral noncritical phase-matching condition of the nonlinear material. The proposed DFG method is generalized to give access to a significant fraction of the molecular fingerprint region by utilizing selenide compounds (e.g., AgGaSe2, CdSe) in addition to ZGP, revealing the remarkable potential of ultrabroadband electronic mid-infrared scanning for numerous spectroscopic applications.

Highly-Efficient Pulsed Mid-Infrared Generation Based on Intracavity Difference Frequency Mixing

We demonstrated highly-efficient pulsed mid-infrared generation based on intracavity difference frequency mixing inside a continuous-wave (CW) PPLN optical parametric oscillator (OPO). The near-infrared pulsed fiber source was located at 1120 nm with variable pulse width and repetition rate. The pump source of OPO was a CW linearly-polarized high-power 1060 nm fiber laser. The 1120 nm and 1060 nm pump beams were simultaneously incident into the OPO cavity. The 1060 nm pump beam built parametric oscillation firstly and generated high-power resonant signal beam inside the cavity. By controlling the PPLN temperature properly, the phase-matched difference frequency generation (DFG) occurred between the signal beam and 1120 nm pulsed pump beam in the PPLN crystal. Finally, the pulsed 1120 nm pump beam was successfully transformed to 3593 nm mid-infrared radiation at every power level and temporal characteristic. Both the slope efficiency and pump-to-idler conversion efficiency of the intracavity DFG process reached over 20%. In addition, the pulse shape of newly generated 3593 nm idler beam was basically the same as 1120 nm pump beam. The experimental results exhibited huge potential in frequency down conversion of low-power laser sources with special temporal characteristics.

Nanosecond optical parametric oscillator with midinfrared intracavity difference-frequency mixing in orientation-patterned GaAs.

We report on efficient midinfrared difference-frequency generation (DFG) in orientation-patterned GaAs by intracavity mixing the signal and idler pulses of a narrowband nanosecond optical parametric oscillator based on periodically poled LiNbO3. The maximum average DFG output power reached 215 mW at 8.15 µm for a repetition rate of 35 kHz. The temperature tuning range spanned over 8026-8710 nm. The maximum overall conversion efficiency from 1 to 8 µm amounted to ∼1.3%.

Highly Efficient Mid-Infrared Generation from Low-Power Single-Frequency Fiber Laser Using Phase-Matched Intracavity Difference Frequency Mixing

In this paper, we demonstrated efficient mid-infrared generation using a low-power 1064 nm single-frequency (SF) fiber laser based on phase-matched intracavity difference frequency generation (DFG) in a continuous-wave (CW) periodically poled lithium niobate (PPLN)-based optical parametric oscillator (OPO). This is the first time that the frequency down conversion of a low-power SF light source has been achieved using intracavity difference frequency mixing. A high power 1018 nm fiber laser was firstly used for building the parametric oscillation and providing the high power resonant signal wave. To realize an efficient DFG process between the SF pump wave and the intracavity signal wave, the temperature of periodically poled lithium niobate (PPLN) crystal was properly adjusted to satisfy the phase-matching conditions. Finally, the low-power 1064 nm SF pump wave was successfully converted to a 3.7 μm mid-infrared wave with a conversion efficiency of 21.6%. The conversion efficiency, to the best of our knowledge, is the highest for SF lasers in DFG processes. Meanwhile, taking advantage of SF laser pumping, a narrow linewidth of 271 pm (5.9 GHz) in the mid-infrared region was achieved without adding any etalons or devices in the cavity.

Frequency downconversion of low-power Raman laser using intracavity difference frequency mixing

We report efficient frequency downconversion of a continuous-wave low-power Raman laser using intracavity difference frequency mixing in a periodically poled lithium niobate (PPLN)-adopted optical parametric oscillator (OPO). The Raman laser was obtained using a 1060-nm fiber laser and 137-m passive fiber. Its central wavelength was fixed at 1111 nm with the power ranging from 320 mW to 5.984 W. The 1060- and 1111-nm pump beams were incident into the OPO at the same time. The high-power 1060-nm pump beam built parametric oscillation first, and the difference frequency generation (DFG) occurred between the low-power Raman laser and intracavity signal laser. The PPLN temperature was properly controlled at 45°C to ensure both the OPO and DFG processes synchronously satisfy phase matching conditions. Benefiting from intracavity high signal power, the Raman laser was successfully converted to the 3560-nm midinfrared radiation under every investigated pump power level. The maximum 3560-nm idler power reached 1.026 W, indicating a 17.4% pump-to-idler slope efficiency and about 15% optical-to-optical conversion efficiency. The comparative experiments also verified that the phase matching conditions were satisfied maximally at 45°C.

Highly efficient continuous-wave mid-infrared generation based on intracavity difference frequency mixing

We report on a new scheme for efficient continuous-wave (CW) mid-infrared generation using difference frequency generation (DFG) inside a periodically poled lithium niobate (PPLN)-based optical parametric oscillator (OPO). The pump sources were two CW fiber lasers fixed at 1018 nm and 1080 nm. One worked as the assisted laser to build parametric oscillation and generate an oscillating signal beam while the other worked at low power ( ${\leqslant}3~\text{W}$ ) to induce DFG between it and the signal beam. The PPLN temperature was appropriately adjusted to enable OPO and DFG to synchronously meet phase-matching conditions. Finally, both low-power 1018 nm and 1080 nm pump beams were successfully converted to $3.1~\unicode[STIX]{x03BC}\text{m}$ and $3.7~\unicode[STIX]{x03BC}\text{m}$ idler beams, respectively. The conversion efficiencies of the 1018 nm and 1080 nm pumped DFG reached 20% and 15%, respectively, while their slope efficiencies reached 19.6% and 15%. All these data were comparable to the OPOs pumped by themselves and never realized before in traditional CW DFG schemes. The results reveal that high-efficiency frequency down-conversion can be achieved with a low-power near-infrared pump source.

Tunable Dual-Wavelength, Continuous-Wave, Mid-Infrared Generation Using Intracavity Difference Frequency Mixing in PPLN-Based Optical Parametric Oscillator

We report on a widely tunable, dual-wavelength, continuous-wave, mid-infrared optical parametric oscillator (OPO), pumped by a dual-wavelength fiber source that was realized by combining two fiber lasers in series. The OPO was designed as a four-mirror singly resonant cavity, in which a periodically poled lithium niobate (PPLN) crystal was used as nonlinear crystal. Two idler emissions with different wavelengths were observed, one being basically fixed at 3140 nm and the other being tunable from 3227 to 3413 nm. Under the maximum pump power, the total idler power was over 6 W and the corresponding pump-to-idler slope efficiency reached 11.4%. The generated signal beam had just one central wavelength around 1600 nm. Through theoretical analysis and experimental verification, it was analyzed that two nonlinear processes, OPO and intracavity difference frequency generation (DFG), simultaneously occurred in the resonant cavity. The experimental phenomena revealed large widened DFG gain spectrum, which was in absolute contrast to simulation results based on classical DFG theory. It indicated great potential in practical application such ultra-wide nonlinear frequency conversion.

Optimization of mid-IR generation from a periodically poled MgO doped stoichiometric lithium tantalate optical parametric oscillator with intracavity difference frequency mixing

We demonstrate 43% slope efficiency for generation of ∼3200 nm light, a wavelength considered to be ideal for laser induced ultrasound generation in carbon fiber reinforced plastic. High slope efficiency was obtained by optimizing crystal lengths, cavity length and mirror reflectivity using a two crystal optical parametric oscillator+difference frequency mixing (OPO+DFM) nonlinear wavelength conversion scheme. Mid-IR output >12 mJ was obtained from a 1064 nm Nd:YAG pump laser with 12 ns pulse width (FWHM) and containing pulse energy of 43 mJ. A compact, single temperature crystal oven is described along with some suggestions for improving the slope efficiency.

Intracavity difference-frequency mixing of optical parametric oscillator signal and idler pulses in BaGa4Se7.

An overall quantum conversion efficiency of 7.8% is achieved by intracavity mixing the signal and idler of a 1.064 μm pumped Rb:PPKTP optical parametric oscillator in BaGa4Se7. In this way, a pulse energy of ∼0.71 mJ is generated at ∼7 μm for a repetition rate of 100 Hz. Tuning of the mid-IR radiation is demonstrated by heating of the Rb:PPKTP crystal.

Wavelength Conversion of Incoherent Broadband Sources by Intracavity Difference Frequency Mixing

We demonstrate a broadband wavelength converting system for incoherent amplified spontaneous emission (ASE) sources. It is based on single-pass difference frequency generation using a periodically poled stoichiometric lithium tantalate crystal placed inside the cavity of a tuneable self-injection-locked diode-pumped Cr <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> :LiCAF laser. A spectral bandwidth of ~ 30 nm around 1550 nm is converted within a tuneable shift of ~ 12 nm. The technique we propose does not only provide ASE sources in new spectral allocations but it also opens up a way to perform spectral shaping.

Mode-Locked Dual-Wavelength Heterolasers for Terahertz Generation via Intracavity Wave Mixing

It is shown that the mid/far-infrared (IR) and THz pulse generation via intracavity difference-frequency mixing in quantum-well dual-wavelength heterolasers can be rather efficient under mode-locking regime for one or both lasing fields even at room temperature. In a simple model we derive an explicit formula for intensity of the generated IR or THz pulse and find that this method is capable of producing picosecond pulses at ≈ 1 GHz repetition rate with the peak power of the order of 1 W and ≤ 0.2 mW at 10 μm and 50 μm wavelengths, respectively.

Difference-frequency pulse generation in quantum well heterolasers

It is shown that mid-to far-infrared (IR) and terahertz (THz) pulse generation via difference-frequency mixing in quantum well (QW) dual-wavelength heterolasers can be rather efficient under the modelocking regime for one or both lasing fields even at room temperature. In such a device, the long-wavelength field is produced in the process of intracavity difference-frequency mixing of two optical fields: continuous wave (CW) and pulsed (or both pulsed), due to the resonant intersubband quantum coherence in QWs, as well as due to the nonresonant second-order semiconductor bulk nonlinearity. The mode-locking regime of the optical generation allows one to significantly enhance the pulsed driving fields in comparison with those under CW operation and, therefore, substantially increase the output difference-frequency power. Within a simple model, an explicit formula for the intensity and shape of the generated IR or THz pulse is derived. It is shown that this method is capable of producing picosecond pulses at a ∼ 1-GHz repetition rate with a peak power of the order of 1 W and ≲0.2 mW at 10 and 50 µm wavelengths, respectively.

Greater than 100% photon-conversion efficiency from an optical parametric oscillator with intracavity difference-frequency mixing

Using 100-ps Nd:YAG pump pulses, we synchronously pump an optical parametric oscillator with intracavity difference-frequency mixing (DFM) between the signal and the idler. The cavity is singly resonant at the signal frequency. The signal, idler, and difference wavelengths are near 1.5, 3.5, and 2.8microm , respectively. Periodically poled lithium niobate is used for both interactions. Results show an 80% enhancement in the idler power-conversion efficiency and an idler photon-conversion efficiency of 110% when the DFM interaction is phase matched. Backconversion of the pump is suppressed when the DFM interaction is phase matched, and pump depletion increases from 65% to 79% at full pump power.

Optical parametric oscillation with intracavity difference-frequency mixing

We report the theoretical analysis of an optical parametric oscillator that contains an intracavity crystal that generates the difference frequency between the signal and the idler produced by the optical parametric oscillator crystal. Phase mismatch in the difference-frequency mixer is considered. The cavity is singly resonant at the signal frequency. We find that high conversion efficiency of the difference and idler frequencies can be achieved over a large dynamic range of the incident pump intensity. A physical example that produces 3.192 μm from a 1.064-μm pump laser is predicted to produce 97.5% quantum efficiency in one polarization component and 197% quantum efficiency in the orthogonal-polarization component for a total power conversion efficiency of 98.2%.