Mirror Loss Research Articles

Planar waveguide to vertical mode couplers covering the visible to IR spectral range

Planar 45o turning mirrors with metal coating embedded in SU8 polymer waveguides enable waveguide to vertical mode coupling across a broad range of wavelengths from the visible to IR. The fabrication of these 2.5D structures is achieved using relatively simple grayscale lithography in thin film resists, compatible with standard planar lithography methods. Mirror losses of <1 dB are measured from 516 -1630 nm, and direct coupling to single mode fibre is achieved.

Optical characterization of a fiber Fabry-Perot cavity: precision measurement of intra-cavity loss, transmittance, and reflectance

We propose and demonstrate a method for characterizing the individual mirror parameters of a fiber Fabry–Perot cavity (FFPC). By measuring the reflection and transmission spectra of the FFPC with an incident laser propagating from the two mirrors of the FFPC and considering several normal or unique losses, the transmittance, reflectance, and intra-cavity loss of the individual mirrors can be determined. Due to the intrinsic limitation of cavity length, traditional powerful methods, such as the cavity ring-down technique, are not applicable to FFPCs for characterizing the parameters of individual mirrors. This scheme provides a dependable method for assessing FFPC mirrors and provides a significant capability for the implementation of strong-coupling cavity quantum electrodynamics based on FFPCs.

Optical bistability and flip-flop function in feedback Fano laser.

Optical bistability has the potential to emulate the capabilities of electrical flip-flops, offering plenty of applications in optical signal processing. Conventional optical bistable devices operate by altering the susceptibility of a nonlinear medium. This method, however, often results in drawbacks such as large device size, high energy consumption, or long switching times. This work proposes an optical bistable device incorporating strong optical feedback into a Fano laser. This leads to multiple stable states and introduces a region of bistability between the inherent Fano mode and a feedback-induced Fabry-Perot mode. Unlike conventional bistable devices, the Fano system exploits strong field localization in a nanocavity to control the properties of one of the laser mirrors. This configuration means that switching states can be achieved by modulating the mirror's loss rather than changing the susceptibility of the active medium. Importantly, modulation can be implemented locally on a nanocavity, bypassing the need to adjust the entire laser system. This leads to fast flip-flop actions with low energy consumption. The feedback Fano laser can be embodied in a compact microscopic structure, thus providing a promising approach towards integrated all-optical computation and on-chip signal processing.

Degradation of Ta2O5 / SiO2 dielectric cavity mirrors in ultra-high vacuum.

In order for optical cavities to enable strong light-matter interactions for quantum metrology, networking, and scalability in quantum computing systems, their mirrors must have minimal losses. However, high-finesse dielectric cavity mirrors can degrade in ultra-high vacuum (UHV), increasing the challenges of upgrading to cavity-coupled quantum systems. We observe the optical degradation of high-finesse dielectric optical cavity mirrors after high-temperature UHV bake in the form of a substantial increase in surface roughness. We provide an explanation of the degradation through atomic force microscopy (AFM), X-ray fluorescence (XRF), selective wet etching, and optical measurements. We find the degradation is explained by oxygen reduction in Ta2O5 followed by growth of tantalum sub-oxide defects with height to width aspect ratios near ten. We discuss the dependence of mirror loss on surface roughness and finally give recommendations to avoid degradation to allow for quick adoption of cavity-coupled systems.

Design of a Compact Hologram System Capable of 3D Lesion Diagnosis in Clinic.

This paper proposes a small-sized hologram system for the 3D imaging of lesions in a clinical environment. In a general hologram system, the distance between the beam-generating device and the screen (400mm) and the size of the screen must be increased proportionally to obtain excellent image quality. However, in a clinical environment, the beam spread distance and screen size must be reduced. This paper proposes a method for reducing the beam divergence distance and screen size for clinical applications. To reduce the beam spread distance and screen size, a beam prism with a 45° refractive index is used to reduce the beam spread distance by 1/3. The direction of the bent light must be adjusted such that it can reach the screen accurately. However, because the reflected light may be refracted owing to the material properties of the mirror and cause loss, this problem can be solved by using a full reflection mirror. The beam spread distance of the designed hologram system is 200mm. The types of lesions obtained from the 3D images of the hologram include the lung, liver, and colon. The image resolution is 300 × 145. If the proposed method is used in a clinical environment, doctors can improve their understanding of the patient quickly and efficiently; thereby, shortening the treatment time. The proposed hologram system is expected to be useful in treatment rooms, operating rooms, and educational programs in medical schools.

Optical characterization of high performance mirrors based on cavity ringdown time measurements with 6 degrees of freedom mirror positioning.

An instrument capable of measuring optical losses, transmission, and the radius of curvature of high reflectivity mirrors is presented. The measurement setup consists of two remote controlled hexapod systems with 6 degrees of freedom placed inside a vacuum enclosure. Mirror loss measurements are performed via the cavity ring-down time method using a linear resonant two-mirror Fabry-Perot cavity configuration. The use of high-precision positioning systems enables cavity loss mapping by transversely scanning the position of the cavity end mirror. Mirror surfaces of up to 30mm in diameter can be scanned, and the cavity length can be tuned by 120mm.

Improved two-mode dynamic model for external-cavity quantum cascade lasers under strong optical feedback

In this paper, we propose an improved model for describing the dynamical behavior and mode selection mechanism of an external-cavity quantum-cascade laser (EC-QCL) when subject to strong optical feedback. Theoretical analysis including the dynamical effective reflectivity model, two-mode rate-equation model and Lang-Kobayashi equations are employed to carry out the laser output characteristics under the multiple feedback effects. A main aspect of the model is that the excess mirror loss caused by the EC effective reflectivity is incorporated into the laser dynamical equations as a nonlinear dynamical variable. Because of accounting the multiple round-trip feedback terms in the EC effective reflectivity, the analysis is valid for the arbitrary feedback regimes from weak to strong levels. Besides the stationary characteristics, the model accurately predicts the dynamical competition between the fundamental laser-cavity mode and the desired EC mode that reflected strongly by external diffraction grating. The results are in well agreement with theoretical and experimental data reported earlier for the EC-QCLs.

Improving Deep Reinforcement Learning With Mirror Loss

Recent years have witnessed the great breakthrough of deep reinforcement learning (DRL) in various AI applications, but the training process needs a very large amount of samples and huge computational costs. To alleviate the low sample efficiency issue, one feasible solution is to improve the state representation learning. We uncover that the agents, trained by the original DRL algorithm, face severe performance degradation in mirrored game environments. As mirror symmetry is an important property of the environment, the poor performance in the mirrored situation indicates that the agents are not fully aware of the essence of the environment. In order to handle this problem and make use of the property to attain better state representation, we propose a novel mirror loss, which serves as an auxiliary module to bring mirror symmetry representation to the DRL agent. It is model-agnostic and prompts the DRL agent to make logically consistent mirrored actions in the mirrored environment. We conduct experiments on OpenAI Gym Atari environments and a more complex RL task, Mahjong AI, and the results demonstrate the efficiency and versatility of our method.

Influence of anti-phase surface relief structure on optical mode and laser output power for 450 nm GaN-based VCSELs

For GaN-based vertical-cavity surface-emitting lasers (VCSELs), single-mode output and high power are difficult to be achieved simultaneously. In this report, we propose increasing the output power while taking the optical field into account by engineering the anti-phase surface relief structure and current aperture size for GaN-based VCSELs. We find that the proposed anti-phase surface relief structure helps to get Gaussian-shaped near-field and far-field patterns for VCSELs. However, such structure suffers from mirror loss and causes decreased laser power. Hence, the current injection aperture size has to be reduced so that the overlap level between the carrier profile and the anti-phase surface relief region can be decreased. Therefore, the VCSEL structure with enhanced laser power and dominating fundamental laser mode can be finally designed.

Terahertz Sources Based on Metrological‐Grade Frequency Combs

AbstractBroadband metrological‐grade frequency comb (FC) synthesizers with a rich number of phase locked modes are the ideal sources for quantum sensing and quantum metrology. At terahertz (THz) frequencies, electrically pumped quantum cascade lasers (QCLs) have shown quantum‐limited frequency noise operation, phase/frequency absolute referencing and self‐starting FC operation, albeit over a rather restricted dynamic range, governed by the nature of the quantum gain media that entangles group velocity dispersion at the different bias points. Here, a technological approach is conceived to achieve FC operation over the entire available gain bandwidth at THz frequencies. The intracavity light intensity of a multistack QCL, inherently showing a giant Kerr nonlinearity, is altered by increasing the mirror losses of its Fabry‐Perot cavity through coating the back facet with an epitaxially‐grown multilayer graphene film. This enables a frequency modulated THz FC showing a proliferation of emitted modes over the entire gain bandwidth and across more than 60% of its operational range, with ≈0.18 mW per mode optical power. The QCL FC is then experimentally characterized to assess its phase coherence, reconstructing its intensity emission profile, instantaneous frequency, and electric field, thus proving its metrological nature.

The effect of humidity on the degradation mechanisms of GaN-based green laser diodes

The degradation behaviors of InAlGaN-based green laser diodes in humid ambients were studied. Nonhermetic laser diodes were aged at three different relative humidity conditions at constant current aging mode. For all nonhermetic devices failure occurred as a result of a large increase in the threshold current and sharp decrease of light output power. The reverse leakage current for the failures did not increase when the threshold current increased and slope efficiency decreased, indicating that the change in the threshold current was a result of a change in internal and/or mirror loss. Besides the threshold current increase, the photoluminescence intensity of the ridge region decreased more than 80% after aging compared with the non-aged companions. The moisture oxidized the p-Ohmic contact region of the semiconductor and caused the increase of the serial resistance. The mirror facet degradation was observed in scanning electron microscope. This led to change in reflectivity and light scattering and eventually to a large shift in the threshold current and slope efficiency of the device.

Scattering loss in precision metrology due to mirror roughness.

Optical losses degrade the sensitivity of laser interferometric instruments. They reduce the number of signal photons and introduce technical noise associated with diffuse light. In quantum-enhanced metrology, they break the entanglement between correlated photons. Such decoherence is one of the primary obstacles in achieving high levels of quantum noise reduction in precision metrology. In this work, we compare direct measurements of cavity and mirror losses in the Caltech 40 m gravitational-wave detector prototype interferometer with numerical estimates obtained from semi-analytic intra-cavity wavefront simulations using mirror surface profile maps. We show a unified approach to estimating the total loss in optical cavities (such as the LIGO gravitational detectors) that will lead towards the engineering of systems with minimum decoherence for quantum-enhanced precision metrology.

Broadband Quantum Dot Superluminescent Diode with Simultaneous Three-State Emission.

Semiconductor superluminescent light-emitting diodes (SLEDs) have emerged as ideal and vital broadband light sources with extensive applications, such as optical fiber-based sensors, biomedical sensing/imaging, wavelength-division multiplexing system testing and optoelectronic systems, etc. Self-assembled quantum dots (SAQDs) are very promising candidates for the realization of broadband SLED due to their intrinsic large inhomogeneous spectral broadening. Introducing excited states (ESs) emission could further increase the spectral bandwidth. However, almost all QD-based SLEDs are limited to the ground state (GS) or GS and first excited state (ES1) emission. In this work, multiple five-QD-layer structures with large dot size inhomogeneous distribution were grown by optimizing the molecular beam epitaxy (MBE) growth conditions. Based on that, with the assistance of a carefully designed mirror-coating process to accurately control the cavity mirror loss of GS and ESs, respectively, a broadband QD-SLED with three simultaneous states of GS, ES1 and second excited-state (ES2) emission has been realized, exhibiting a large spectral width of 91 nm with a small spectral dip of 1.3 dB and a high continuous wave (CW) output power of 40 mW. These results pave the way for a new fabrication technique for high-performance QD-based low-coherent light sources.

Ultra-low threshold lasing through phase front engineering via a metallic circular aperture

Semiconductor lasers with extremely low threshold power require a combination of small volume active region with high-quality-factor cavities. For ridge lasers with highly reflective coatings, an ultra-low threshold demands significantly suppressing the diffraction loss at the facets of the laser. Here, we demonstrate that introducing a subwavelength aperture in the metallic highly reflective coating of a laser can correct the phase front, thereby counter-intuitively enhancing both its modal reflectivity and transmissivity at the same time. Theoretical and experimental results manifest a decreasing in the mirror loss by over 40% and an increasing in the transmissivity by 104. Implementing this method on a small-cavity quantum cascade laser, room-temperature continuous-wave lasing operation at 4.5 μm wavelength with an electrical consumption power of only 143 mW is achieved. Our work suggests possibilities for future portable applications and can be implemented in a broad range of optoelectronic systems.

Investigation of the noise characteristics of vertical-cavity surface-emitting laser with a rhomboidal oxide current aperture for use in a Cs-based compact atomic magnetometer

The possibility of using vertical-emitting lasers with intracavity contacts (IC-VCSEL) and a rhomboidal oxide current aperture for creating a non-zero magnetic field optically pumped atomic magnetometers (OPM) with a 133Cs vapor cell for magnetoencephalographic (MEG) systems were demonstrated. Relative intensity noise (RIN) and polarization resolved RIN of the IC-VCSEL in the 895 nm range with different mirror losses (linewidth) in the frequency range from 1 Hz to 100 kHz were experimentally investigated. Lasers with low mirror loss (narrow linewidth) have polarization resolved RIN comparable to amplitude noise. For IC-VCSEL with an output optical power of 0.8 mW and a linewidth of 55 MHz, the noise level measured is 148 dB/Hz in 1 Hz bandwidth at 40 kHz frequency. The ultimate sensitivity of OPM based on two-beam MZ-scheme with studied VCSELs was estimated as ~ 11 fT/sqrt(Hz)sqrt. Keywords: vertical-cavity surface-emitting laser (VCSEL), magnetoencephalographic (MEG) systems, optically pumped atomic magnetometers (OPAM), relative intensity noise (RIN), polarization-resolved RIN.

Engineering the spectral bandwidth of quantum cascade laser frequency combs.

Quantum cascade lasers (QCLs) facilitate compact optical frequency comb sources that operate in the mid-infrared and terahertz spectral regions, where many molecules have their fundamental absorption lines. Enhancing the optical bandwidth of these chip-sized lasers is of paramount importance to address their application in broadband high-precision spectroscopy. In this work, we provide a numerical and experimental investigation of the comb spectral width and show how it can be optimized to obtain its maximum value defined by the laser gain bandwidth. The interplay of nonoptimal values of the resonant Kerr nonlinearity and cavity dispersion can lead to significant narrowing of the comb spectrum and reveals the best approach for dispersion compensation. The implementation of high mirror losses is shown to be favorable and results in proliferation of the comb sidemodes. Ultimately, injection locking of QCLs by modulating the laser bias around the round trip frequency provides a stable external knob to control the frequency-modulated comb state and recover the maximum spectral width of the unlocked laser state.

Approaches to tuning the exceptional point of PT-symmetric double ridge stripe lasers.

Electrically injected Parity-time (PT)-symmetric double ridge stripe semiconductor lasers lasing at 980 nm range are designed and measured. The spontaneous PT-symmetric breaking point or exceptional point (EP) of the laser is tuned below or above the lasing threshold by means of varying the coupling constant or the mirror loss. The linewidth of the optical spectrum of the PT-symmetric laser is narrowed, compared with that of traditional single ridge (SR) laser and double ridge (DR) laser. Furthermore, the far field pattern of the PT-symmetric laser with EP below the lasing threshold is compared with that of the PT-symmetric laser with EP above the lasing threshold experimentally. It is found that when the laser start to lase, the former is single-lobed while the latter is double-lobed. when the current continues to increase, the former develops into double lobe directly while the latter first develops into single lobe and then double lobe again.

Quantum Noise in a Fabry-Perot Interferometer Including the Influence of Diffraction Loss of Light

The DECi-hertz Interferometer Gravitational wave Observatory (DECIGO) is designed to detect gravitational waves at frequencies between 0.1 and 10 Hz. In this frequency band, one of the most important science targets is the detection of primordial gravitational waves. DECIGO plans to use a space interferometer with optical cavities to increase its sensitivity. For evaluating its sensitivity, diffraction of the laser light has to be adequately considered. There are two kinds of diffraction loss: leakage loss outside the mirror and higher-order mode loss. These effects are treated differently inside and outside of the Fabry-Perot (FP) cavity. We estimated them under the conditions that the FP cavity has a relatively high finesse and the higher-order modes do not resonate. As a result, we found that the effects can be represented as a reduction of the effective finesse of the cavity with regard to quantum noise. This result is useful for optimization of the design of DECIGO. This method is also applicable to any FP cavities with a relatively small beam cut and the finesse sufficiently higher than 1.

Coplanar electrode directly modulated lasers with different cavity length

AbstractA ridge‐waveguide (RW) directly modulated laser (DML) with a coplanar electrode structure is proposed and measured in this paper. Since utilizing the coplanar electrode can significantly reduce parasitic capacitance, the frequency responses of DMLs exhibit a quite low roll‐off. Shortening the cavity length L is attractive to enhance the high‐speed performance of DMLs, but it will increase the mirror loss and the chip resistance at the same time. We analyzed the static and dynamic characteristics of 200‐μm, 250‐μm, and 300‐μm lasers experimentally and theoretically to investigate the impact of L on the coplanar‐electrode DML. Shortening the cavity length can improve the relaxation oscillation frequency fr effectively. The 200‐μm DML achieves a bandwidth of 15.3 GHz. The fr is the most important limiting factor for the laser to obtain a wide bandwidth. Decreasing L and increasing the differential gain are promising for the further improvement of fr. However, shortening L causes a more severe thermal effect and the deterioration of the linearity. This trade‐off must be considered in the future study.

Исследование шумовых характеристик вертикально-излучающих лазеров с ромбовидной токовой апертурой для применения в компактном квантовом цезиевом магнитометре

The possibility of using vertical-emitting lasers with intracavity contacts (IC-VCSEL) and a rhomboidal oxide current aperture for creating a non-zero magnetic field optically pumped atomic magnetometers (OPM) with a 133Cs vapor cell for magnetoencephalographic (MEG) systems were demonstrated. Relative intensity noise (RIN) and polarization resolved RIN of the IC-VCSEL in the 895 nm range with different mirror losses (linewidth) in the frequency range from 1 Hz to 100 kHz were experimentally investigated. Lasers with low mirror loss (narrow linewidth) have polarization resolved RIN comparable to amplitude noise. For IC-VCSEL with an output optical power of 0.8 mW and a linewidth of 55 MHz, the noise level measured is 148 dB/Hz in 1 Hz bandwidth at 40 kHz frequency. The ultimate sensitivity of OPM based on two-beam MX scheme with studied VCSELs was estimated as ~ 11 fT/√Hz.