Superluminescent Light Research Articles

Custom chromatic confocal microscope for surface profilometry

In this work, a surface profiling system is introduced, employing a custom-designed chromatic confocal (CC) distance sensor. Today’s commercial sensors based on the CC principle have a major drawback of effectively coupling light from the illumination source into the optical imaging system, mainly due to etendue mismatch. Consequently, a substantial amount of light is lost, significantly reducing the overall SNR of the system. This limitation restricts its applicability primarily to highly reflective surfaces. To address this issue, an etendue-matched optical system is proposed, that incorporates an innovative superluminescent light source. Through this approach, the system achieves 4 orders of magnitudes higher illumination intensity concentrated within the focus spot compared to existing devices in the current market.

Scalable Printing of Metal Nanostructures through Superluminescent Light Projection.

Direct printing of metallic nanostructures is highly desirable but current techniques cannot achieve nanoscale resolutions or are too expensive and slow. Photoreduction of solvated metal ions into metallic nanoparticles is an attractive strategy because it is faster than deposition-based techniques. However, it is still limited by the resolution versus cost tradeoff because sub-diffraction printing of nanostructures requires high-intensity light from expensive femtosecond lasers. Here, this tradeoff is overcome by leveraging the spatial and temporal coherence properties of low-intensity diode-based superluminescent light. The superluminescent light projection (SLP) technique is presented to rapidly print sub-diffraction nanostructures, as small as 210nm and at periods as small as 300nm, with light that is a billion times less intense than femtosecond lasers. Printing of arbitrarily complex 2D nanostructured silver patterns over 30µm×80µm areas in 500ms time scales is demonstrated. The post-annealed nanostructures exhibit an electrical conductivity up to 1/12th that of bulk silver. SLP is up to 480 times faster and 35 times less expensive than printing with femtosecond lasers. Therefore, it transforms nanoscale metal printing into a scalable format, thereby significantly enhancing the transition of nano-enabled devices from research laboratories into real-world applications.

InAlN cladding implementation in green superluminescent diodes and lasers

We demonstrate state-of-the-art superluminescent light emitting diodes emitting in the green spectral range with InAlN n-type claddings. The implementation of an InAlN optical blocking layer for suppressing modal substrate leakage leading to high farfield quality is first reported. An optimized structure by optical mode simulations featuring a bottom n-type InAlN cladding layer is then presented. Compared to conventional AlGaN cladding based devices, the electro-optical performance, optical confinement, and modal substrate leakage are greatly improved. Performance improvements in green laser diode devices are also presented.

BB84 quantum key distribution transmitter utilising broadband sources and a narrow spectral filter.

The secure nature of Quantum Key Distribution (QKD) protocols makes it necessary to ensure that the single photon sources are indistinguishable. Any spectral, temporal or spatial discrepancy between the sources would lead to a breach in the security proofs of the QKD protocols. Traditional, weak-coherent pulse implementations of polarization-based QKD protocols have relied on identical photon sources obtained through tight temperature control and spectral filtering. However, it can be challenging to keep the temperature of the sources stable over time, particularly in a real-world setting, meaning photon sources can become distinguishable. In this work, we present an experimental demonstration of a QKD system capable of achieving spectral indistinguishability, over a 10°C range, using a combination of broadband sources, super-luminescent light emitting diodes (SLEDs), along with a narrow band-pass filter. The temperature stability could be useful in a satellite implementation, where there may be temperature gradients over the payload, particularly on a CubeSat.

All-optical reservoir computing system based on polarization dynamics

Reservoir computing (RC) is a simplified recurrent neural network and can be implemented by using a nonlinear system with delay feedback, thus it is called delay-based RC. Various nonlinear nodes and feedback loop structures have been proposed. Most of existing researches are based on the dynamical responses in intensity of the nonlinear systems. There are also a photoelectric RC system based on wavelength dynamics and an all-optical RC based on the phase dynamics of a semiconductor laser with optical feedback, as well as so-called polarization dynamics of a vertical cavity surface emitting laser (VCSEL). However, these VCSEL-RCs actually are based on the intensity dynamics of two mutually orthogonal polarization modes, or polarization-resolved intensity dynamics. The RC based on rich dynamical responses in polarization has not yet been found. A semiconductor optical amplifier (SOA) fiber ring laser can produce rich dynamical states in polarization, and is used in optical chaotic secure communication and distributed optical fiber sensing. To further expand the application of polarization dynamics of the SOA fiber ring laser and open up a new direction for the research of optical RC neural network, an all-optical RC system based on polarization dynamics of the ring laser is proposed. The ring laser is used as the reservoir, and the SOA as the nonlinear node. After the input signal is masked according to a synchronization scheme, it is injected into the reservoir by intensity modulation for a continuous wave generated by a superluminescent light emitting diode (SLED). The dynamical response in polarization of the ring laser is detected by a polarizer and a photodetector. The influences of the SOA operation current, output power of the SLED and attenuation of a variable optical attenuator (VOA) in the fiber loop on the polarization dynamic characteristic (mainly referring to the output degree of polarization) of the laser are analyzed experimentally. The fading memory and nonlinear response of the RC system based on the polarization dynamic response and intensity dynamic response are compared experimentally. The influences of output power of the SLED and attenuation of the VOA on fading memory, consistency and separation of the RC system based on the two kinds of dynamic responses are investigated experimentally. Thus the range of the VOA attenuation is determined. The network performance of the polarization dynamics RC system is evaluated by processing a Santa Fe time series prediction task and a multi-waveform recognition task. The normalized mean square error can be as low as 0.0058 for the time series prediction task, and the identification rate can be as high as 100% for the recognition task under the appropriate system parameters and only 30 virtual nodes. The experimental results show that the polarization dynamics RC system has good prediction performance and classification capability, which are comparable to the existing RC system based on intensity dynamics of the ring laser. The system can be expected to process two tasks in parallel when the polarization dynamics and intensity dynamics are used at the same time.

Optimization of end-fire coupling between an LED mid-IR light source and SiNx optical waveguides for spectroscopic sensing

We design and optimize an optical coupling structure between a superluminescent light emitting diodes (SLED) and SiNx waveguides for the optical sensing, in which an efficient coupling between these waveguides is challenging due to significant cross-sectional difference. From FDTD investigation, we identify the coupling scheme that is suitable to directly couple light from a SLED into a relatively-thin SiNx waveguide suitable for optical sensing based on the evanescent field absorption. The coupling structure is systematically investigated and optimized in terms of refractive indices, coupler’s geometrical parameters, and fabrication variations with corresponding detection performance. The coupling scheme could be instrumental in enabling the use of SLED technology in Si-based photonic integrated circuits for sensing applications.

A miniaturized, low-cost and portable fiber Bragg grating interrogation system for remote monitoring

A miniaturized, low-cost, 4-channel fiber Bragg grating (FBG) interrogation system for real-time remote monitoring is presented in this paper. A superluminescent light emitting diode (SLED) as the broadband light source combined with an interrogation module and an optical switch is applied in the system. The moving average method is employed to remove noise effects, and then Gaussian fitting method is used to realize the interrogation of FBG central wavelength with high precision. On software design, the WebSocket network protocol is introduced to establish communication between data processing core and webpages to achieve cross-platform remote access through the browser without user’s software installation. The proposed interrogation system not only displays the reflected spectrum and demodulated central wavelength in real time on the webpage, but also realizes the data storage and channel switch remotely. The interrogator for remote monitoring of FBG sensors proposed in this paper has been proved experimentally with the interrogation resolution 0.1 pm, the precision of 5 pm for the FBG central wavelength, interrogation stability up to ± 1.1 pm, and interrogation frequency of 4 kHz.

A Miniature Fiber-Optic Silicon Cantilever-Based Acoustic Sensor Using Ultra-High Speed Spectrum Demodulation

A silicon cantilever-based fiber-optic acoustic sensor (FOAS) is presented in this work. A rectangular cantilever is fabricated upon a silicon-on-insulator (SOI) wafer using a micro-electro-mechanical system (MEMS). The length, width and thickness of the silicon cantilever are 530 μm, 200 μm and 3 μm, respectively. The resonant frequency of the silicon cantilever is 14820 Hz with a sensitivity of 950 nm/Pa. An ultra-high speed absolute cavity length demodulation method is adopted using a complementary metal oxide semiconductor (CMOS) spectrometer and an 850 nm superluminescent light emitting diode (SLED). A modified Buneman frequency estimation and total phase demodulation algorithm is adopted. The frequency response curve shows a relatively flat trend from 20 Hz to 13 kHz. The sensor exhibits good linearities at different frequencies while the applied acoustic pressure is increased from 0 Pa to 2.5 Pa. Experimental results indicate that the minimum detectable pressure (MDP) of the proposed FOAS is calculated to be 25.68 μPa/Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sup> at the frequency of 13 kHz. The silicon cantilever-based FOAS achieves broad operating bandwidth, high sensitivity, small size, and good stability.

The role of spontaneous and piezoelectric polarization fields on the spectral and power characteristics of InxGa1-xN/GaN superluminescent light emitting diodes

The role of spontaneous and piezoelectric polarization fields on the spectral and power characteristics of InxGa1-xN/GaN superluminescent light emitting diodes

Numerical Simulation on Third-Harmonic Generation of Super-Luminescent Light by Mixing Broadband and Narrowband Lasers

Numerical Simulation on Third-Harmonic Generation of Super-Luminescent Light by Mixing Broadband and Narrowband Lasers

Modeling power and linewidth of quantum dot superluminescent light emitting diode

A model for quantum dot (QD) superluminescent light emitting diode (SLED) is presented to study an optical power output and linewidth over a wide range of injection currents. The analysis is based on the photon and carrier rate equations including the effects of homogeneous and inhomogeneous broadening, carrier escape process, and high-current heating. The model is validated using experimental data available from the literature. The results show non-monotonic variations of the output optical power and linewidth of the SLED with the injection current density. It is seen that there exists an optimum injection current density for which the power–linewidth product becomes maximum for a given device length of the QD SLED.

Wavelength tunable picosecond pulse bunch laser around 3 µm through PPMgLN –based DFG with temperature insensitive high power scaling

We report a bunch-mode-operated, wavelength-tunable, temperature-insensitive high power, mid-infrared (MIR) laser based on difference frequency generation (DFG) in a periodically poled Magneoisum-oxide doped Lithium Niobate (PPMgLN) chip. Two synchronized fiber lasers each with master oscillator power amplifier (MOPA) structures are utilized as pump and signal sources for the DFG. The pump laser is seeded by a mode-locked Yb fiber laser around 1030 nm with fundamental repetition rate of 16.32 MHz and pulse duration of 8.5 ps. Each pulse of the pump seed is multiplied by a factor of 16, forming the pulse bunch, and then amplified to 27.8 W. A super-luminescent light emitting diode (SLED) around 1580 nm is triggered by the first pulses of the pump bunch and adopted as the signal seed, which is amplified to 4 W afterwards. The pulse duration and spectral bandwidth of the signal are ~ 5 ns and 50 nm, respectively, covering the total duration of the pump bunch in the time domain and signal wavelength tuning range in the spectral domain, respectively, enabling wavelength-tuning DFG with temperature-insensitive power scaling. High power pulse bunch MIR laser can always be realized within the crystal temperature range from 25 to 95 °C with corresponding idler wavelength from 3.03 μm to 2.84 μm. The highest idler power of 4.6 W is achieved at 3.0 μm with pump-to-idler conversion efficiency of ~ 16.5%. It is believed that such a bunch-mode-operated, mid-infrared ps laser source with high output power within the whole temperature-wavelength tuning range are of great value to the evaluation of wavelength dependent laser bio-ablation.

Long-Term Reliability of Fiber-Optic Current Sensors

We present studies on the long-term reliability of interferometric fiber-optic current sensors (FOCS) for use in electric power transmission systems. Accelerated ageing tests are performed on crucial optical sensor components and a three-phase sensor system is subjected to an extended field trial. The sensor components under test include the sensor’s superluminescent light emitting diode light source, the integrated-optic phase modulator and various passive components such as fiber couplers, fiber polarizers, polarization-maintaining fiber connectors, and fiber coatings. The components are exposed to accelerated ageing conditions for extended periods of time, i.e., temperature cycling (between −25 °C and 65 °C for up to 15000 cycles), constant high temperature at dry conditions (up to 115 °C for up to 20000 h), and damp heat (85 % relative humidity at 85 °C for up to 7900 h). Crucial component parameters such as the source wavelength and polarization extinction ratios are repeatedly measured as a function of temperature at defined intervals during the ageing periods and examined for potential drift of component failures. The field trial is carried out for a three-phase FOCS system integrated into 420 kV double-chamber circuit breakers over a period of more than three years. The sensor signals are compared to the signals of conventional current transformers. In addition, the evolution of various operational parameters such as the light source power is continuously recorded. The results prove a high degree of reliability of modern FOCS systems.

Quantum cascade superluminescent light emitters with high power and compact structure

Quantum cascade (QC) superluminescent light emitters (SLEs) have emerged as desirable broadband mid-infrared (MIR) light sources for growing number of applications in areas like medical imaging, gas sensing and national defense. However, it is challenging to obtain a practical high-power device due to the very low efficiency of spontaneous emission in the intersubband transitions in QC structures. Herein a design of ~5 μm SLEs is demonstrated with a two-phonon resonance-based QC active structure coupled with a compact combinatorial waveguide structure which comprises a short straight part adjacent to a tilted stripe and to a J-shaped waveguide. The as-fabricated SLEs achieve a high output power of 1.8 mW, exhibiting the potential to be integrated into array devices without taking up too much chip space. These results may facilitate the realization of SLE arrays to attain larger output power and pave the pathway towards the practical applications of broadband MIR light sources.

Single-transverse-mode broadband InAs quantum dot superluminescent light emitting diodes by parity-time symmetry.

Parity-time (PT) symmetry breaking in counterintuitive gain/loss coupled waveguide designs is numerically and theoretically investigated. The PT symmetry mode selection conditions are determined theoretically. Single-transverse-mode broadband InAs quantum dot (QD) superluminescent light emitting diodes (SLEDs) are fabricated and characterized; the PT symmetric broad-area SLEDs contain laterally coupled gain and loss PT- symmetric waveguides. Single-transverse-mode operation is achieved by parity-time symmetry breaking. The broadband SLEDs exhibit a uniform Gaussian-like emission spectrum with the 3-dB bandwidth of 110 nm. Far-field characteristics of the coupled waveguide SLEDs exhibit a single-lobe far-field pattern when the gain and loss waveguides are biased at the injection current of 600 mA and 60 mA, respectively.

Mid-infrared broadband superluminescent light emitter arrays

Mid-infrared (MIR) room-temperature (RT) and continuous-wave (CW) broadband quantum cascade superluminescent light emitters (QCSLEs) have emerged as ideal broadband light sources for a number of applications of biomedical imaging, security inspection, and gas detection. It is quite challenging to attain a RT-CW output power up to milliwatt level due to the very low efficiency of the spontaneous emission in the intersubband transitions in QCSLEs. In this work, for the first time to the best of our knowledge, a compact light emitter array is realized by integrating several single emitters exhibiting a very high RT-CW power of 2.4mW, which is attributed to the sufficient low reflectivity provided by the waveguide structure that includes three sections with a short straight part adjacent to a tilted stripe and to a J-shaped waveguide, and the two-phonon resonance QC active structure. This advancement is certainly a big step forward to the applications of broadband light sources towards MIR photonics.

Modeling and Simulation of the Intrinsic Function of a Photonic Switch

In this paper, I provide modeling and simulation results which show that a photonic bistable switch can be realized by the application of two superluminescent light emitting diodes to a configuration of three 10μm radius micro-ring resonators. The 130nm silicon on insulator rib structure is adopted for the simulations. The three 10μm radius rings have a resonant wavelength of 1.5487μm. The on or off state, at a single output port, is realized by the application of a superluminescent diode at either, one or the other, of the two input ports. The bistable output intensity, at the ring resonant wavelength, is shown as a single normalized peak representing a 1 or the “on” condition. The single peak is realized by the application of the “on” superluminescent diode. The “off” or the 0 condition, realized by the application of the “off” superluminescent diode, is exhibited at the output by the introduction of an optical transparency at the single peak resonant wavelength. The minimum of the optical transparency can be adjusted by changing the coupling gap distances of the design. The transparency mimum introduces an error detection feature whereby a failure of the “off” excitation superluminecent diode results in a zero or noise floor output. The transparency minimum can be increased to raise the off state minimum above the measurement apparatus noise floor. The output of the switch can be detected using a simple photodiode intensity detection method.

Room-temperature quantum cascade superluminescent light emitters with wide bandwidth and high temperature stability.

The realization of room-temperature (RT) mid-infrared (MIR) broadband light sources is fundamentally interesting and highly desirable for a number of applications. Recently, superluminescent light emitters (SLEs) based on quantum cascade (QC) structures have emerged as excellent candidates among mid-infrared broadband light sources. However, it is challenging to achieve RT-QCSLEs due to the very low efficiency of the spontaneous emission in the intersubband transitions. Here, we demonstrate the realization of a set of ~5 μm RT-SLEs under continuous wave (CW) or quasi-CW (10% duty circle) operation by using a two-phonon resonant QC active region and monolithic integrated waveguide structures. In addition, with the design of an inclined tapered cavity, the SLEs exhibit high milliwatt power, large spectral width of more than 200 cm-1 and good temperature characteristic. These demonstrated results are believed to be a big step forward to the applications of broadband MIR semiconductor light sources.

Optical Fiber Demodulation System with High Performance for Assessing Fretting Damage of Steam Generator Tubes.

In order to access the fretting damage of the steam generator tube (SGT), a fast fiber Fabry-Perot (F-P) non-scanning correlation demodulation system based on a super luminescent light emitting diode (SLED) was performed. By demodulating the light signal coming out from the F-P force sensor, the radial collision force between the SGT and the tube support plate (TSP) was interrogated. For higher demodulation accuracy, the effects of the center wavelength, bandwidth, and spectrum noise of SLED were discussed in detail. Specially, a piezoelectric ceramic transducer (PZT) modulation method was developed to get rid of the interference of mode coupling induced by different types of fiber optics in the demodulation system. The reflectivity of optical wedge and F-P sensor was optimized. Finally, the demodulation system worked well in a 1:1 steam generator test loop and successfully demodulated a force signal of 32 N with a collision time of 2 ms.

Ytterbium-Doped Fiber Amplifiers Seeded With Superluminescent Light Emitting Diode

We demonstrate multi-stage ytterbium-doped fiber amplifiers (YDFAs) seeded with a superluminescent light emitting diode (SLED). Laser pulses with four pulse durations, such as 350-ps, 6-ns, 55-ns, and 183-ns (full width half maximum), have been studied. The peak powers reach 332, 95, 119, and 92 kW for the four types of laser pulses, respectively. The 350-ps laser pulses are generated through gain switching. Thanks to the broadband optical spectrum of the seed laser from the SLED, the stimulated Brillouin scattering is avoided. The configuration of such YDFAs seeded by an SLED with adjustable laser pulse durations (from picosecond to nanosecond) is desirable for many industrial applications.