Low-loss Cavity Research Articles

Few-emitter lasing in single ultra-small nanocavities.

Lasers are ubiquitous for information storage, processing, communications, sensing, biological research and medical applications. To decrease their energy and materials usage, a key quest is to miniaturise lasers down to nanocavities. Obtaining the smallest mode volumes demands plasmonic nanocavities, but for these, gain comes from only a single or few emitters. Until now, lasing in such devices was unobtainable due to low gain and high cavity losses. Here, we demonstrate a form of 'few emitter lasing' in a plasmonic nanocavity approaching the single-molecule emitter regime. The few-emitter lasing transition significantly broadens, and depends on the number of molecules and their individual locations. We show this non-standard few-emitter lasing can be understood by developing a theoretical approach extending previous weak-coupling theories. Our work paves the way for developing nanolaser applications as well as fundamental studies at the limit of few emitters.

Monolithic ultrashort-cavity DBR gain-switched thulium doped fiber laser made by high-precision femtosecond laser direct-writing

We demonstrate a novel monolithic ultrashort-cavity distributed Bragg reflector (DBR) gain-switched thulium doped silica fiber laser. Commercial heavily thulium doped non-polarization-maintaining silica fiber was selected as the gain medium. Monolithic linear laser resonator, which includes a pair of uniform fiber Bragg gratings (FBGs) with a space of 1.0-mm inbetween, was inscribed in the thulium fiber by femtosecond-laser point-by-point (PbP) writing method. The reflectivity of either FBG is close to 100 % and the effective cavity length is calculated to be 3.5 mm. Such an ultrashort-DBR-cavity fiber laser exhibited high Q factor and low cavity loss, allowing the efficient utilization of pump power within the millimeter-long cavity. Single-longitudinal, linearly polarized, gain-switched TDFL at 2003 nm, with a recorded shortest pulse duration of 4.3 ns, a repetition rate of 50 kHz, a maximum peak power of 500 W, and single pulse energy of 2.14 μJ, has been realized, benefitted from the ultrashort cavity length.

Efficient laser operation with low loss Nd3+-doped phosphate glass fiber under direct pumping

For the first time, we have demonstrated efficient 1054 nm laser output with low loss Nd3+-doped phosphate glass fibers at room temperature using an 888 nm laser diode as the direct pumping source. Therein, the home-made Nd3+-doped phosphate glass fiber is the key for laser performance improvement. The transmission loss of the large-core single-cladding Nd3+-doped phosphate fibers prepared by the novel peeled-off isolated stacked extrusion method is as low as 1.1 dB/m at ∼1 μm. On this basis, a low-loss Fabry-Perot cavity is constructed by precisely controlling the relative distance between the fiber end facet and the cavity mirror with a microscope. The record-breaking optical-to-optical efficiency of Nd3+-doped phosphate glass fiber lasers, 50.2% is achieved with the maximum output power of 6.71 W. During laser operation, the output laser power does not roll over, even at high pump powers. The output laser power is limited only by the available pump power. This work will lay a solid foundation to the high-power fiber laser fabrication, and will find many applications in the future.

Micro-nano fiber-assisted active photoacoustic spectroscopy for gas sensing.

We report on the development of all-fiber active photoacoustic spectroscopy, where active photoacoustic effect is generated by embedding a micro-nano fiber inside a fiber laser resonator to exploit the evanescent field of the high intracavity power. Acetylene detection at 1530.37 nm was selected for gas sensing demonstration. With a small diameter of 1.1 µm, the tapped fiber exploited ∼20% intracavity power for the evanescent-wave photoacoustic excitation, while only introduced a low intrinsic cavity loss of 0.08 dB. Our sensor achieved a minimum detection limit of 1 ppm at an integration time of 10 s, which can be improved to 73 ppb at 1000 s benefited from the high system stability. The sensing dynamic range was determined to be more than five orders. This spectroscopic technique combines fiber laser, photoacoustic spectroscopy, and fiber evanescent-wave absorption to achieve gas sensing with high flexibility, low optical noise, and easy optical alignment. Current limitations were discussed in detail to explore feasible ways to improve the performance in response time, dynamic range and sensitivity.

Biaxially Textured YBa2Cu3O7−x Microwave Cavity in a High Magnetic Field for a Dark-Matter Axion Search

In the search for axion dark matter, superconducting rf technology is important for fabricating microwave cavities with high $Q$ factors, but the presence of a strong external magnetic field leads to technical difficulties. This study presents a polygonal design with tapes of high-temperature superconductor to realize a superconducting resonant cavity that shows no considerable degradation in magnetic fields up to 8 Tesla. This approach could impact engineering solutions to improve dark-matter detection and other research areas requiring low cavity loss in a strong magnetic field.

Ultrafast soliton delivered by miniaturized mode-locker with MoTe2 and core-expanded fiber.

Thermally expanded core (TEC) technology is an effective method of high-power fiber lasers. Miniaturization is also a major challenge for high-power lasers. We have proposed a miniaturized mode-locker based on TEC fiber and MoTe2-polyvinyl alcohol (PVA) film. The proposed mode-locker is consisting of two TEC ferrules, a piece of MoTe2-PVA film and a ceramic sleeve. The length of the proposed device is about 20 mm, and its outer diameter is about 2 mm. The relations between heating time, heating temperature, and mode field diameter (MFD) have been numerically simulated. The bending loss with respect to MFD has also been analyzed. The simulation results have revealed the trade-off relation between maximal tolerable intensity and low cavity loss, which means that there is an optimal MFD corresponding to optimal heating time and heating temperature. The proposed mode-locker has been applied in an integrated fiber laser, which has emitted ultrafast soliton with 3 times intensity larger than that of conventional sandwiched-type saturable absorber. The proposed mode-locker and fiber laser will find important applications in laser processing, laser ranging, and optical communication.

Hybrid integrated external cavity laser with a 172-nm tuning range

Chip-scale lasers with a wide tunable range and a narrow linewidth have rich applications in optical communications, sensing, and light detection and ranging systems. External cavity lasers (ECLs) have the advantage of a wide tuning range and a narrow linewidth compared with their counterparts such as distributed feedback lasers and distributed Bragg reflector lasers because the passive filter separated from the gain section can be widely tuned and the low-loss external cavity extends the photon lifetime. However, so far, the tuning range achieved by the chip-scale ECLs is in the range of 80–120 nm, smaller than that of the bulk free-space ECL (typically >160 nm). The bottleneck in reaching an ultrabroad tuning range for chip-scale ECLs lies in the unsuppressed side modes of the external cavity. Here, we demonstrate a hybrid-integrated ECL consisting of a broadband III–V reflective semiconductor optical amplifier and a low-loss Si3N4 wavelength-selective reflector. A record 172-nm lasing wavelength tuning range together with a more than 40 dB side-mode suppression ratio is achieved beyond the free spectral range of the Vernier ring filter due to the enhanced lasing mode selection from a tunable Sagnac loop reflector. The Si3N4 platform enables a low-loss external cavity, facilitating laser linewidth reduction below 4 kHz over the full tuning range. The on-chip output power reaches 26.7 mW at a wavelength of 1550 nm.

Effect of linewidth enhancement factor on the generation of optical vortices in a class-A degenerate cavity semiconductor laser.

The dynamical behavior of a one-dimensional ring array of lasers generated in a class-A degenerate cavity semiconductor laser is numerically investigated. The class-A behavior of the laser is obtained by considering a low-loss vertical external cavity surface emitting laser (VECSEL), in which a telescope and a mask allow us to control the geometry and the linear nearest-neighbour coupling between the lasers. The behavior of the lasers is simulated using coupled rate equations, taking the influence of the Henry factor into account. It is shown that the ring array of lasers exhibits multistability. Moreover, by comparison with a class-B semiconductor laser, it is proved that the class-A nature of the laser makes it more robust to the increase of the Henry factor when it comes to generating topological charge carrying arrays of lasers, thus opening new perspectives of application for such lasers.

Temperature and Curvature Measurement Based on Low Cavity Loss FLRD Technology

In this study, we proposed a fiber loop ring-down (FLRD) based method for simultaneous temperature and curvature measurements. A tapered single-mode fiber (TSMF) and a TSMF covered with polydimethylsiloxane (PDMS-TSMF) were used as curvature and temperature sensors, respectively. Owing to the low insertion loss of these sensors, erbium-doped fiber amplifiers were not required to compensate for the intensity of the pulsed light in the FLRD. The insertion loss of PDMS-TSMF decreased as the ambient temperature increased, which decreased the slope of the peak envelope of the ring-down pulse. The results show that the linear relationship between the pulse envelope slope and temperature varied from 1.237 dB/(km °C) to 0.397 dB/(km °C) in the temperature range of 40–70 °C. The insertion loss of TSMF increased with the increase in curvature. The slope-curvature relationship was 51.013 dB/(km m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> ) from 2.706 to 4.364 m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> . The proposed quasi-distributed sensing system has the advantages of low cavity loss, high sensitivity, low cost, and independent measurement of two variables simultaneously. It has potential value in practical engineering applications, especially in building structural health detection, disaster warning.

Exotic Coupling Between Plasmonic Nanoparticles Through Geometric Configurations

Coupled plasmonic systems are of great interest and have many applications such as information processing and sensing. By choosing proper geometric configurations of coupled plasmonic systems, one can obtain various optical properties. However, some interesting and important effects could not be described by earlier methods. We develop an improved method for coupled plasmonic nanoparticle systems that maps geometric configurations to optical properties more accurately. With the improved method, we realize a low-loss cavity of metallic nanoparticles through a proper geometric configuration, and we find a limit to the loss in the metallic nano-cavity. We also use this method to realize an exceptional point and exceptional nexus in a hybrid plasmonic system. Finally, we predict asymmetric coupling, which leads to chirality and directional energy transfer.

Metal substrate modulation for realizing a more compact whispering-gallery microcavity

The compact cavity is always conducive to the integration and application of the devices based on whispering gallery modes. However, it is a challenge to strike a balance between the pursuit of a smaller cavity and higher quality. Here, we propose a low-loss cavity by combining a microsphere with a metal substrate, thereby compressing the cavity size by 24%. In addition, the substrate-modulated cavity is found to have a higher confinement factor and Purcell factor, showing the good prospect for realizing high-performance compact lasers.

10 W injection-locked single-frequency continuous-wave titanium:sapphire laser

High-power tunable lasers with good longitudinal and transverse modes are fundamental tools for exploring quantum physics. Here we report a high-power continuous-wave injection-locked titanium:sapphire laser with a low-loss cavity configuration, where only a laser crystal was installed in the laser cavity. Although the transverse mode was affected by a thermal lens formed in the laser crystal, the focal length of the thermal lens could be shifted via the temperature of the laser crystal holder or the pump power. As a result, we found a condition that 10 W single-frequency oscillation with a good transverse mode and a slope efficiency of 51% were achieved.

45 nm broadband continuously tunable semiconductor disk laser

A broadband continuously tunable semiconductor disk laser is reported in this paper. The active region of gain chip is composed of InGaAs multiple quantum wells with resonant periodic gain structure, and its fluorescence peak wavelength is around 965 nm. Using the wideband characteristics of the quantum wells in gain chip, along with the simple linear cavity that is formed by a high reflectivity external mirror, the laser has a low cavity loss and a wide tuning range. The continuously tunable laser wavelength can be obtained by inserting birefringent filters with different thickness into the cavity. When the thickness of the birefringent filter is 2 mm, the wavelength tuning range of the laser is 45 nm, the maximum output power is 122 mW, and the beam quality &lt;i&gt;M&lt;/i&gt;&lt;sup&gt;2&lt;/sup&gt; factors in the &lt;i&gt;X-&lt;/i&gt; and the &lt;i&gt;Y-&lt;/i&gt;directions are 1.00 and 1.02, respectively. The temperature characteristics of the surface-emitting spectra of gain chip and the narrowing effect of birefringent filter on laser linewidth h are also discussed.

Sub-kHz linewidth, hybrid III-V/silicon wavelength-tunable laser diode operating at the application-rich 1647-1690 nm.

The wavelength region about of 1650 nm enables pervasive applications. Some instances include methane spectroscopy, free-space/fiber communications, LIDAR, gas sensing (i.e. C2H2, C2H4, C3H8), surgery and medical diagnostics. In this work, through the hybrid integration between an III-V optical amplifier and an extended, low-loss wavelength tunable silicon Vernier cavity, we report for the first time, a III-V/silicon hybrid wavelength-tunable laser covering the application-rich wavelength region of 1647-1690 nm. Room-temperature continuous wave operation is achieved with an output power of up to 31.1 mW, corresponding to a maximum side-mode suppression ratio of 46.01 dB. The laser is ultra-coherent, with an estimated linewidth of 0.7 kHz, characterized by integrating a 35 km-long recirculating fiber loop into the delayed self-heterodyne interferometer setup. The laser linewidth is amongst the lowest in hybrid/heterogeneous III-V/silicon lasers.

Dislocation-Free and Atomically Flat GaN Hexagonal Microprisms for Device Applications.

III-nitrides are considered the material of choice for light-emitting diodes (LEDs) and lasers in the visible to ultraviolet spectral range. The development is hampered by lattice and thermal mismatch between the nitride layers and the growth substrate leading to high dislocation densities. In order to overcome the issue, efforts have gone into selected area growth of nanowires (NWs), using their small footprint in the substrate to grow virtually dislocation-free material. Their geometry is defined by six tall side-facets and a pointed tip which limits the design of optoelectronic devices. Growth of dislocation-free and atomically smooth 3D hexagonal GaN micro-prisms with a flat, micrometer-sized top-surface is presented. These self-forming structures are suitable for optical devices such as low-loss optical cavities for high-efficiency LEDs. The structures are made by annealing GaN NWs with a thick radial shell, reforming them into hexagonal flat-top prisms with six equivalents either m- or s-facets depending on the initial heights of the top pyramid and m-facets of the NWs. This shape is kinetically controlled and the reformation can be explained with a phenomenological model based on Wulff construction that have been developed. It is expected that the results will inspire further research into micron-sized III-nitride-based devices.

Hundred megahertz microwave photonic filter based on a high Q silicon nitride multimode microring resonator

Narrowband microwave photonic filters based on a microring resonator are difficult to achieve because low cavity loss and low coupling loss should be satisfied simultaneously. Here, a high Q (∼2.6×106) multimode microring resonator is proposed to achieve an ultra-narrow band tunable microwave photonic filter. Combining the ultra-low loss of the silicon nitride waveguide and the ultra-low coupling coefficients of the multimode ring resonator, very narrow optical bandwidths between 72.5 MHz to 275 MHz were obtained for different order modes, which match the simulation results well. Furthermore, by introducing the two switchable modulation methods supported by the dual-drive Mach–Zehnder modulator, we achieved a narrowband passband/stopband switchable microwave photonic filter, whose 3 dB bandwidths are 180 MHz and 120 MHz, respectively. The filter frequency can be tuned from 2 ∼ 18 GHz by altering the laser wavelength, and a high out of band RF rejection ratio about 27 dB was obtained for the passband filter due to the high-quality factor. Besides, a high RF rejection ratio of about 51 dB was achieved for the stopband filter by using the RF cancellation technology.

Frequency-Dependent Squeezing for Advanced LIGO.

The first detection of gravitational waves by the Laser Interferometer Gravitational-Wave Observatory (LIGO) in 2015 launched the era of gravitational-wave astronomy. The quest for gravitational-wave signals from objects that are fainter or farther away impels technological advances to realize ever more sensitive detectors. Since 2019, one advanced technique, the injection of squeezed states of light, is being used to improve the shot-noise limit to the sensitivity of the Advanced LIGO detectors, at frequencies above ∼50 Hz. Below this frequency, quantum backaction, in the form of radiation pressure induced motion of the mirrors, degrades the sensitivity. To simultaneously reduce shot noise at high frequencies and quantum radiation pressure noise at low frequencies requires a quantum noise filter cavity with low optical losses to rotate the squeezed quadrature as a function of frequency. We report on the observation of frequency-dependent squeezed quadrature rotation with rotation frequency of 30Hz, using a 16-m-long filter cavity. Anovel control scheme is developed for this frequency-dependent squeezed vacuum source, and the results presented here demonstrate that a low-loss filter cavity can achieve the squeezed quadrature rotation necessary for the next planned upgrade to Advanced LIGO, known as "A+."

High coherence superconducting microwave cavities with indium bump bonding

Low-loss cavities are important in building high-coherence superconducting quantum computers. Generating high-quality joints between parts is crucial for the realization of a scalable quantum computer using the circuit quantum electrodynamics (cQED) framework. In this paper, we adapt the technique of indium bump bonding to cQED to realize high-quality superconducting microwave joints between chips. We use this technique to fabricate compact superconducting cavities in the multilayer microwave integrated quantum circuit (MMIQC) architecture and achieve single photon quality factors over 300 × 106 or single-photon lifetimes approaching 5 ms. To quantify the performance of the resulting seam, we fabricate microwave stripline resonators in multiple sections connected by different numbers of bonds, resulting in a wide range of seam admittances. The measured quality factors combined with the designed seam admittances allow us to bound the conductance of the seam at gseam≥2×1010/(Ω m). Such a conductance should enable the construction of micromachined superconducting cavities with a quality factor of at least a billion. These results demonstrate the capability to construct very high-quality microwave structures within the MMIQC architecture.

Demonstration of interferometer enhancement through EPR entanglement.

The recent series of gravitational-wave (GW) detections by the Advanced LIGO and Advanced Virgo observatories launched the new field of GW astronomy. As the sensitivity of GW detectors is limited by quantum noise of light, concepts from quantum metrology have been adapted to increase the observational range. Since 2010, squeezed light with reduced quantum noise has been used for improved sensitivity at signal frequencies above 100 Hz. However, 100 m long optical filter resonators would be required to also improve the sensitivity at lower frequencies, adding significant cost and complexity. Here we report on a proof-of-principle setup of an alternative concept that achieves the broadband noise reduction by using Einstein-Podolsky-Rosen (EPR) entangled states instead. We show that the desired sensitivity improvement can then be obtained with the signal-recycling resonator that is already part of current observatories, providing the viable alternative to high-cost filter cavities.

Alexandrite: an attractive thin-disk laser material alternative to Yb:YAG?

Yb:YAG thin-disk (TD) technology has enabled construction of laser/amplifier systems with unprecedented average/peak power levels, and has become the workhorse of many scientific investigations. On the other hand, for some applications, the narrow emission bandwidth of Yb:YAG limits its potential, and the search for alternative broadband TD gain media with suitable thermo-optomechanical parameters is ongoing. The alexandrite gain medium has a broad emission spectrum centered around 750 nm, possesses thermomechanical strength that even outperforms Yb:YAG, and has unique spectroscopic properties enabling efficient laser operation even at elevated temperatures. In this work, we have numerically investigated the power scaling potential of continuous-wave (cw) alexandrite lasers in TD geometry for the first time. Using a detailed laser model, we have compared the potential cw laser performance of Yb:YAG, Ti:Sapphire, Cr:LiSAF, Cr:LiCAF, and alexandrite thin-disk lasers under similar conditions and show that among the investigated transition metal-doped gain media, alexandrite is the best alternative to Yb:YAG in power scaling studies at room temperature. Our analysis further demonstrates that potentially Ti:Sapphire is also a good alternative TD material, but only at cryogenic temperatures. However, in comparison with Yb:YAG, the achievable laser gain is relatively low for both alexandrite and Ti:Sapphire, which then requires usage of low-loss cavities with small output coupling for efficient cw operation.