Distributed Bragg Reflector Cavity Research Articles

Polarization-Maintaining Single-Frequency Fiber Laser With Quadruple Wavelengths at the C-Band

In this paper, a polarization-maintaining quad-wavelength (QW) single-frequency fiber laser (SFFL) at the C-band with the novel and compact structure is demonstrated. Applying the rate equations and multiple coupled nonlinear Schrodinger equations, the theoretical model of QW fiber lasers is established, and then the optical spectra, temporal evolution, and stability of the QW fiber laser are analyzed numerically. Based on the theoretical analysis, a QW-SFFL having an ultrashort linear distributed Bragg reflector cavity is proposed. Utilizing a wideband fiber Bragg grating coupled with a dual-channel polarization maintaining fiber Bragg grating as the wavelength selection and a 15-mm-long Er 3+ /Yb 3+ co-doped phosphate fiber as the gain medium has realized a robust QW laser with the wavelength spacing of 0.4 nm. All the wavelengths maintain the single-frequency operation independently with the linewidths of 20 kHz. This type of compact SFFLs with quadruple wavelengths is widely adapted to lidar systems and fiber sensing.

Oxide-Confined VCSELs for High-Speed Optical Interconnects

The electrically pumped vertical-cavity surface-emitting laser (VCSEL) was first demonstrated with metal cavities by Iga (1979); however, the device threshold current was too high. Distributed Bragg reflector cavities proposed by Scifres and Burnham (1975) were adopted to improve the optical cavity loss. Yet, it was not a practical use until the discovery of the native oxide of AlGaAs and the insertion of quantum wells to provide simultaneous current and optical confinement in semiconductor laser by Holonyak and Dallesasse (1990). Later, the first “low-threshold” oxide-confined VCSEL was realized by Deppe (1994) and opened the door of commercial application for a gigabit energy-efficient optical links. At present, we demonstrated that the oxide-confined VCSELs have advanced error-free data transmission [bit-error rate (BER) $\le 10^{-12}$ ]to 57 Gb/s at 25 °C and 50 Gb/s at 85 °C, and also demonstrated that the pre-leveled 16-quadrature amplitude modulation orthogonal frequency-division multiplexing data were achieved at 104 Gbit/s under back-to-back transmission with the received error vector magnitude, SNR, and BER of 17.3%, 15.2 dB, and $3.8\times 10^{-3}$ , respectively.

Fabrication and characterization of a wide-bandgap and high-[formula omitted] terahertz distributed-Bragg-reflector micro cavities

We report high-Q and wide-bandgap distributed-Bragg-reflector (DBR) ‘micro’ cavities with a single mode frequency around 1 THz by stacking alternatively predefined quartz discs and gaskets. Semi-finished precision crystalline quartz discs for commercial quartz oscillators with two different thicknesses were carefully chosen to form quarter-wavelength Bragg mirrors and DBR cavities. Characterizations of DBR cavities by terahertz time-domain spectroscopy and continuous-wave transmission measurements agree well with transfer-matrix-method (TMM) simulations. The made DBR cavities yielded a quality factor as high as 1, 560 at 0.907 THz with a bandgap of 420 GHz from 0.68 to 1.10 THz. The stacking method was adopted to form DBR cavities with an even higher Q factor by using quarter-wavelength high-resistivity silicon slabs and gaskets. Such stacking method allows for reliable and versatile integration of terahertz Bragg mirrors and DBR cavities with active terahertz devices such as modulators, emitters and detectors. Limited by the penetration depth of terahertz field in Bragg mirrors, the effective mode volume is slightly less than half of the resonant wavelength. Further reduction of the mode volume could be achieved by field enhancement and/or localization within the cavity.

1.3 μm single photon emission from InAs/GaAs quantum dots

Single-photon emitters are crucial for the applications in quantum communication, random number generation and quantum information processing. Self-assembled InAs/GaAs quantum dots (QDs) have demonstrated to have singlephoton emission with high extraction efficiency, single-photon purity, and photon indistinguishability. Thus they are considered as the promising deterministic single-photon emitters. To extend the emission wavelength of InAs/GaAs QDs to telecom band, several methods have been developed, such as the strain engineered metamorphic quantum dots, the use of strain reducing layers and the strain-coupled bilayer of QDs. In fact, it is reported on single-photon emissions based on InAs/InP QDs with an emission wavelength of 1.55μm, but it is difficult to combine such QDs with a high-quality distributed Bragg reflector (DBR) cavity because the refractive index difference between InP and InGaAsP is too small to obtain a DBR cavity with high quality factor. Here we investigate 1.3μm single-photon emissions based on selfassembled strain-coupled bilayer of InAs QDs embedded in micropillar cavities. The studied InAs/GaAs self-assembled QDs are grown by molecular beam epitaxy on a semi-insulating (100) GaAs substrate through strain-coupled bilayer of InAs QDs, where the active QDs are formed on the seed QDs capped with an InGaAs layer, and two-layer QDs are vertically coupled with each other. In such a structure the emission wavelength of QDs can be extended to 1.3μm. The QDs with a low density of about 6×10<sup>8</sup> cm<sup>-2</sup> are embedded inside a planar 1-<sup>λ</sup> GaAs microcavity sandwiched between 20 and 8 pairs of Al<sup>0.9</sup>Ga<sup>0.1</sup>As/GaAs as the bottom and top mirror of a DBR planar cavity, respectively. Then the QD samples are etched into 3μm diameter micropillar by photolithography and dry etching. The measured quality factor of studied pillar cavity has a typical value of approximately 300. Photoluminescence (PL) spectra of QDs at a temperature of 5 K are examined by using a micro-photoluminescence setup equipped with a 300 mm monochromator and an InGaAs linear photodiode array detector. A diode laser with a continuous wave or a pulsed excitation repetition rate of 80 MHz and an excitation wavelength of 640 nm is used to excite QDs through an near-infrared objective (<sup>NA</sup> 0.5), and the PL emission is collected by the same objective. The time-resolved PL of the QDs is obtained by a time-correlated single photon counting. The second-order correlation function is checked by a Hanbury-Brown and Twiss setup through using ID 230 infrared single-photon detectors.<br/>In summary, we find that the 1.3μm QD exciton lifetime at 5 K is measured to be approximately 1 ns, which has the same value as the 920 nm QD exciton lifetime. The second-order correlation function is measured to be 0.015, showing a good characteristic of 1.3μm single photon emission. To measure the coherence time, i.e., to perform highresolution linewidth measurements, of the QDs emitted at the wavelength of 920 and 1300 nm, we insert a Michelson interferometer in front of the spectrometer. The obtained coherence time for 1.3μm QDs is 22 ps, corresponding to a linewidth of approximately 30μeV. Whereas, the coherence time is 216 ps for 920 nm QDs, corresponding to a linewidth of approximately 3μeV. Furthermore, both emission spectral lineshapes are different. The former is of Gaussian-like type, while the latter is of Lorentzian type.

A deterministic quantum dot micropillar single photon source with >65% extraction efficiency based on fluorescence imaging method

We report optical positioning of single quantum dots (QDs) in planar distributed Bragg reflector (DBR) cavity with an average position uncertainty of ≈20 nm using an optimized photoluminescence imaging method. We create single-photon sources based on these QDs in determined micropillar cavities. The brightness of the QD fluorescence is greatly enhanced on resonance with the fundamental mode of the cavity, leading to an high extraction efficiency of 68% ± 6% into a lens with numerical aperture of 0.65, and simultaneously exhibiting low multi-photon probability (g(2)(0) = 0.144 ± 0.012) at this collection efficiency.

Single photon extraction from self-assembled quantum dots via stable fiber array coupling

We present a direct fiber output of single photons from self-assembled quantum dots (QDs) realized by a stable fiber array-QD chip coupling. The integration of distributed Bragg reflector cavity and the etching of micropillar arrays isolate QDs and enhance their normal emission. The matched periods and mismatched diameters of the pillar array and the single-mode fiber array with Gaussian-shaped light spots enable a large alignment tolerance and a stable, efficient (i.e., near-field), and chip-effective (i.e., parallel) coupling of single QD emission, as compared to the traditional “point-based” coupling via a confocal microscope, waveguide, or fiber. The single photon counting rate at the fiber end reaches 1.87 M counts per second (cps) with a time correlation g2(0) of 0.3 under a saturated excitation, and 485 K cps with a g2(0) of 0.02 under a weak excitation, demonstrating a nice “all-fiber” single-photon source.

A tunable fiber-coupled optical cavity for agile enhancement of detector absorption

Maximizing photon absorption into thin active structures can be the limiting factor for photodetector efficiency. In this work, a fiber-coupled tunable cavity is demonstrated, designed to achieve close to unity absorption of photons into a thin film superconducting nanowire single photon detector (SNSPD). A technique for defining a stable cavity between the end of a telecommunications optical fiber and a reflective substrate is described and realized. Cavity resonances are demonstrated both through the tuning of input wavelength and cavity length. The resulting optical cavity can tune the resonant absorption in situ over a wavelength range of 100 nm. This technique is used to maximize the single photon absorption into both a back-side-coupled Au mirror SNSPD and a front-side-coupled distributed Bragg reflector cavity SNSPD. The system detection efficiency (SDE) is limited by imperfections in the thin films, but in both cases we demonstrate an improvement of the SDE by 40% over bare fiber illumination.

Frequency- and intensity-noise suppression in Yb3+-doped single-frequency fiber laser by a passive optical-feedback loop.

The frequency and intensity noise of an Yb3+-doped single-frequency distributed Bragg reflector (DBR) fiber laser are effectively reduced by a simple, passive optical-feedback loop (POFL), which consists of only two optical couplers. The feedback loop, which has resonance with the high reflective grating of the DBR laser and relative long optical path compared to the DBR cavity, results in narrower linewidth and lower relative intensity noise (RIN) in the feedback signal. The RIN of relaxation oscillation is reduced by 20dB from -99.9dB/Hz @ 993 kHz to -119.4dB/Hz @ 192 kHz, and the frequency noise was suppressed at frequencies higher than 1 kHz, with a maximum reduction of about 30 dB from 10 kHz to 100 kHz, which results in a spectral linewidth compression from 3.96 kHz to 540 Hz. Even after one fiber amplification stage, the noise did not increase significantly, and a spectral linewidth well below 1 kHz were also achieved at output power of 10W.

Resonant pumped erbium-doped waveguide lasers using distributed Bragg reflector cavities.

This Letter reports on an optical pumping scheme, termed resonant pumping, for an erbium-doped distributed feedback (DFB) waveguide laser. The scheme uses two mirrors on either side of the DFB laser, forming a pump cavity that recirculates the unabsorbed pump light. Symmetric distributed Bragg reflectors are used as the mirrors and are designed by matching the external and internal quality factors of the cavity. Experimental demonstration shows lasing at an optical communication wavelength of around 1560 nm and an improvement of 1.8 times in the lasing efficiency, when the DFB laser is pumped on-resonance.

High performance organic distributed Bragg reflector lasers fabricated by dot matrix holography.

We report distributed Bragg reflector (DBR) polymer lasers fabricated using dot matrix holography. Pairs of distributed Bragg reflector mirrors with variable mirror separations are fabricated and a novel energy transfer blend consisting of a blue-emitting conjugated polymer and a red-emitting one is spin-coated onto the patterned substrate to complete the device. Under optical pumping, the device emits sing-mode lasing around 622 nm with a bandwidth of 0.41 nm. The working threshold is as low as 13.5 μJ/cm² (~1.68 kW/cm²) and the measured slope efficiency reaches 5.2%. The distributed feedback (DFB) cavity and the DBR cavity resonate at the same lasing wavelength while the DFB laser shows a much higher threshold. We further show that flexible DBR lasers can be conveniently fabricated through the UV-imprinting technique by using the patterned silica substrate as the mold. Dot matrix holography represents a versatile approach to control the number, the size, the location and the orientation of DBR mirrors, thus providing great flexibility in designing DBR lasers.

Short monolithic dual-wavelength single-longitudinal-mode DBR phosphate fiber laser.

We propose and demonstrate a 5-cm-long monolithic dual-wavelength single-longitudinal mode distributed Bragg reflector (DBR) all-phosphate fiber laser. Strong UV-induced fiber Bragg gratings are directly written in highly Er/Yb codoped phosphate fiber. The separation between gratings is selected as 1cm to only excite two longitudinal modes in the DBR cavity. By exploiting the spatial hole burning effect and the polarization hole burning effect, stable narrow-linewidth dual-wavelength lasing emission with 38pm wavelength spacing and a total emitted power of 2.8mW is obtained from this DBR fiber laser. A microwave signal at 4.58GHz is generated by the heterodyne detection of the dual-wavelength laser.

Unidirectional Transmission Based on a Passive PT Symmetric Grating With a Nonlinear Silicon Distributed Bragg Reflector Cavity

A silicon waveguide consisting of passive parity-time (PT) symmetry optical potentials connected with a distributed Bragg reflector (DBR)-based resonator is proposed to achieve nonreciprocal waveguide transmission. The unidirectional reflectionless waveguide implementing PT symmetry is discussed by consistent coupling mode theory and finite-difference time-domain (FDTD) simulation results. Due to the high field confinement in the DBR cavity, transmission of a light pulse through the device is analyzed in the nonlinear optical regime using FDTD simulations. The combination of the nonlinear DBR cavity and the passive PT symmetric grating results in unidirectional transmission in the silicon waveguide, while still keeping the reflection minimum in the desired direction. The numerical simulation shows an extinction ratio of about 20 dB at the telecom wavelength of around 1550 nm.

Measurement and modification of biexciton-exciton time correlations

Photons which are generated in a two-photon cascade process have an underlying time correlation since the spontaneous emission of the upper level populates the intermediate state. This correlation leads to a reduction of the purity of the photon emitted from the intermediate state. Here we characterize this time correlation for the biexciton-exciton cascade of an InAs/GaAs quantum dot. We show that the correlation can be reduced by tuning the biexciton transition in resonance to a planar distributed Bragg reflector cavity. The enhanced and inhibited emission into the cavity accelerates the biexciton emission and slows down the exciton emission thus reduces the correlation and increases the purity of the exciton photon. This is essential for schemes like creating time-bin entangled photon pairs from quantum dot systems.

Investigation of Axial Strain Effects on Microwave Signals from a PM-EDF Short Cavity DBR Laser for Sensing Applications

The effects of axial strain on beating frequency from a short cavity polarization-maintaining erbium-doped fiber (PM-EDF) based distributed Bragg reflector (DBR) laser were investigated theoretically and experimentally for the first time. This type of single-mode DBR fiber laser based ultrasensitive sensor has been extensively developed for measuring kinds of measurands, but the cross-sensitivity of axial strain was usually ignored. A DBR fiber laser with an effective cavity length of $\sim$ 1 cm formed by a pair of FBGs written on a PM-EDF was fabricated for demonstration. This laser operated in dual-polarization single-longitudinal mode stably. The frequency of the beating signal generated by two orthogonal polarizations was found to be proportional to the axial strain applied on the cavity. A linear strain sensitivity of 0.640 $\hbox{GHz/m}\varepsilon$ was obtained, with a fiber birefringence of around 8.65e-5. For the fiber with larger birefringence, the sensitivity to the axial strain will be increased. So the effects of axial strain should be considered when the structure is utilized as a high-resolution sensor, especially for a polarization-maintaining system. The single-mode DBR laser made of high birefringent fibers also has a potential application in frequency tunable microwave generation.

Monolithic distributed Bragg reflector cavities in Al 2O 3 with quality factors exceeding 10 6

The design, fabrication, and characterization of surface relief Bragg gratings integrated with aluminum oxide ridge waveguides are reported. After patterning a photoresist layer by laser interference lithography, uniform gratings with a depth of ∼120 nm and a period of 507 nm were etched into the SiO 2 top cladding. The grating length varied between 1.25 mm and 4.75 mm. The grating-induced loss was 0.08 ± 0.01 dB/cm, while the maximum grating reflectivity exceeded 99%. These values enabled the realization of monolithic distributed Bragg reflector cavities with finesse up to 147 and quality factors of more than 1.0 × 10 6. The measured performance agrees very well with predictions based on coupled mode theory.

A digital-alloy AlGaAs/GaAs distributed Bragg reflector for application to [formula omitted] surface emitting laser diodes

A distributed Bragg reflector (DBR) utilizing the digital alloy Al 0.9Ga 0.1As was grown by using the molecular beam epitaxy (MBE) method for application in 1.3 μm optical communications and compared to the analog-alloy AlGaAs/GaAs DBR. The transmission electron microscopic (TEM) image showed a highly abrupt boundary of AlAs and GaAs, which supports the formation of a digital-alloy Al 0.9Ga 0.1As layer. The measurement showed that the digital-alloy AlGaAs/GaAs DBR had similar reflection spectra with enhanced uniform distribution over the whole substrate surface compared to the analog-alloy one. In the digital-alloy Al 0.9Ga 0.1As/GaAs DBR cavity, the reflection dip position was measured at around 1273 nm and the standard deviation of the distribution of the reflection dip was 1.31 nm in wavelength over 1/4 of a three-inch wafer.

Nonlinear optical phase shift obtained from two-level atoms confined in a planar microcavity

We investigate the nonlinear optical phase shift obtained from a thin atomic layer confined in a distributed Bragg reflector (DBR) microcavity with reflection geometry. The optical response of the atom-cavity system is numerically analyzed using finite-difference time-domain method with the optical Bloch equations. The optimal position of atomic layer, at which a maximal phase shift of π is realized, drastically changes with the quality factor Q of the cavity. We show that for high Q the maximal phase shift of π can be obtained anywhere in the cavity field independently of atomic layer position. This result is in contrast to that obtained from a one-dimensional atom model in the limit of bad cavity, where a maximal phase shift of π is obtained only at the antinode of the cavity field. We also show that the independence of phase shift on atom position realized in high-Q regime is due to an interference effect in the surface layers of the DBR cavity.

An improved model for computing the reflectivity of a AlAs/GaAs based distributed bragg reflector and vertical cavity surface emitting laser

In this paper, reflectivity of a Distributed Bragg Reflector (DBR) has been computed by considering the effects of changes of wavelength on the changes of the refractive index of the materials of DBR layers. The intrinsic losses of the materials have been included in the computation of the reflectivity of the DBR. It has been found that the effect of change of the wavelength on the refractive index of the DBR materials reduces the Full Width Half Maxima (FWHM) of the stop band significantly which is expected to improve the laser characteristics. If the FWHM is reduced, the thickness of the active layer of a VCSEL can also be reduced which will further reduce the threshold current of the device. It has been found that the intrinsic losses of the materials have a significant effect on the reflectivity of a DBR. It has also been found that peak reflectivity of a 20 pair AlAs/GaAs DBR reduces by 0.2% after including the intrinsic losses (with a value of the intrinsic losses α = 10 cm−1).

N-Type Doping to an Active-Short Cavity DBR Laser to Expand Its Continuous Tuning Range

We propose the use of a short-cavity distributed Bragg reflector (DBR) laser with a modified active DBR structure, which has an n-type-doped active single quantum well, and that achieves fast wavelength switching that is potentially applicable for optical packet switching networks. The laser exhibits 6.4-nm continuous tuning by moderating a steep gain change at a low DBR injection current. Furthermore, 32-nm continuous tuning has also been achieved for an integrated laser with an eight-channel array.

Effect of lateral periodicity on the optical response of a quantum well in a distributed Bragg reflector cavity: A simplified description via the Green function of a cavity polariton

In order to examine the previous proposal by two of the authors [A. D'Andrea and L. Pilozzi, Phys. Status. Solidi A 164, 77 (1997); L. Pilozzi and A. D'Andrea, Phys. Rev. B 61, 4771 (2000); Microelectron. Eng. 51/52, 211 (2000)] to enhance the Rabi splitting of a distributed Bragg reflector cavity polariton via introducing a lateral grating in the cavity layer, the dispersion curves of cavity polaritons (including the leaky modes of the cavity) with the diffraction effect due to the lateral grating, have been calculated by using the Green function of a cavity polariton. The analytic form of the Green function, arising from the separability of exciton polarizability, plays an essential role to simplify the calculation of complicated dispersion curves. The comparison of the calculated dispersion curves and reflectivity spectra justifies the interpretation of the enhanced Rabi splitting as due to the coupling of a quantum well exciton and a mode originating from the mixture of a cavity mode and diffracted grating modes, with more weight on the latter. This establishes a different way of enhancing radiation-matter coupling by the use of Bragg diffracted beams.