Wavelength-tunable Light Source Research Articles

Photoacoustic microscopy for real-time monitoring of near-infrared optical absorbers inside biological tissue.

We developed a high-speed optical-resolution photoacoustic microscopy (OR-PAM) system using a high-repetition-rate supercontinuum (SC) light source and a two-axes Galvano scanner. The OR-PAM system enabled real-time imaging of optical absorbers inside biological tissues with excellent excitation wavelength tunability. In the near-infrared (NIR) wavelength range, high-speed OR-PAM faces limitations due to the lack of wavelength-tunable light sources. Our study aimed to enable high-speed OR-PAM imaging of various optical absorbers, including NIR contrast agents, and validate the performance of high-speed OR-PAM in the detection of circulating tumor cells (CTCs). A high-repetition nanosecond pulsed SC light source was used for OR-PAM. The excitation wavelength was adjusted by bandpass filtering of broadband light pulses produced by an SC light source. Phantom and in vivo experiments were performed to detect tumor cells stained with an NIR contrast agent within flowing blood samples. The newly developed high-speed OR-PAM successfully detected stained cells both in the phantom and in vivo. The phantom experiment confirmed the correlation between the tumor cell detection rate and tumor cell concentration in the blood sample. The high-speed OR-PAM effectively detected stained tumor cells. Combining high-speed OR-PAM with molecular probes that stain tumor cells in vivo enables in vivo CTC detection.

Multi-color stimulated Raman scattering with a frame-to-frame wavelength-tunable fiber-based light source.

We present multi-color imaging by stimulated Raman scattering (SRS) enabled by an ultrafast fiber-based light source with integrated amplitude modulation and frame-to-frame wavelength tuning. With a relative intensity noise level of -153.7 dBc/Hz at 20.25 MHz the light source is well suited for SRS imaging and outperforms other fiber-based light source concepts for SRS imaging. The light source is tunable in under 5 ms per arbitrary wavelength step between 700 cm−1 and 3200 cm−1, which allows for addressing Raman resonances from the fingerprint to the CH-stretch region. Moreover, the compact and environmentally stable system is predestined for fast multi-color assessments of medical or rapidly evolving samples with high chemical specificity, paving the way for diagnostics and sensing outside of specialized laser laboratories.

Inverse design of a single-frequency diffractive biosensor based on the reporter cleavage detection mechanism.

Vertically interrogated porous silicon (PSi) interferometric biosensors have shown high potential for sensing bio-molecules as they combine high detection sensitivity with simplicity of fabrication, functionalization, optical coupling, and interfacing with microfluidic systems. However, most interferometric sensor designs require either broadband or wavelength-tunable light sources as well as wide-angle detection schemes, increasing their complexity and cost for point-of-care biosensing applications. The limit of detection of such sensors is also constrained by the small size and low refractive index of biological molecules, making it hard to detect very low concentrations of pathogens. In this work, we use a large-scale computational "inverse design" technique to demonstrate a single-frequency, fixed-angle PSi-based biosensor, which exploits a recently developed high-contrast reporter cleavage detection (HCCD) technique. The HCCD sensors detect high-index reporter cleavage events instead of low-index target analyte capture events as typical for traditional label-free optical biosensors. We use the inverse design approach to discover an optimal configuration of a PSi biosensor that makes use of the extended achievable range of cleavage-induced PSi effective index variations and can be interrogated at a single frequency and at a fixed angle. The optimized design in the form of a one-dimensional PSi grating exhibits the change in the reflectance up to 55 % at the interrogation angle of 12∘ and wavelength of 600 nm, which is caused by cleavage of Au nanoparticle reporters initially occupying 2% of the sensor surface area. The maximum possible change in reflectance is predicted to be 222 % (for a two-dimensional freeform design not amenable to fabrication). This demonstration may pave the way for developing new or redesigned conventional interferometric and colorimetric point-of-care biosensor systems in combination with the cleavage-based detection schemes.

Off-axis integrated cavity output spectroscopy for real-time methane measurements with an integrated wavelength-tunable light source

Off-axis integrated cavity output spectroscopy (OA-ICOS) is a sensitive technique for trace gas sensing. A wavelength modulated off-axis integrated cavity output spectroscopy (WM-OA-ICOS) based CH4 detection system was presented in this paper. An integrated near-infrared (NIR) light source consisting of a digital temperature controller, a current modulator and a 1.65 μm distributed feedback (DFB) laser was developed. A 50.8 mm long assembly Fabry–Perot (F-P) cavity was designed and used as the gas chamber. Wavelength modulation spectroscopy (WMS) was employed to improve the signal to noise ratio (SNR) of the system and a LabVIEW-based orthogonal lock-in amplifier was developed to extract the second harmonic (2f) signal. The experimental values of the optimum modulation parameters were determined and CH4 measurements were carried out to demonstrate the effectiveness and sensitivity of the sensor system. A quantitative characterization of long-term stability and detection sensitivity is presented. Based on Allan variance analysis, the limit of detection is 10.5 ppm when the averaging time is 30 s which can be improved easily by changing the cavity lens with higher reflectivity or increase the distance between the cavity mirrors. The sensor system was insensitive to optical alignment and can be easily serviced on site. Furthermore, this system can be further integrated by using self-developed lock-in amplifier based on our previous work, which enables the system to have good prospects in industrial field.

Polychromatic digital holographic microscopy: a quasicoherent-noise-free imaging technique to explore the connectivity of living neuronal networks.

.Significance: Over the past decade, laser-based digital holographic microscopy (DHM), an important approach in the field of quantitative-phase imaging techniques, has become a significant label-free modality for live-cell imaging and used particularly in cellular neuroscience. However, coherent noise remains a major drawback for DHM, significantly limiting the possibility to visualize neuronal processes and precluding important studies on neuronal connectivity.Aim: The goal is to develop a DHM technique able to sharply visualize thin neuronal processes.Approach: By combining a wavelength-tunable light source with the advantages of hologram numerical reconstruction of DHM, an approach called polychromatic DHM (P-DHM), providing OPD images with drastically decreased coherent noise, was developed.Results: When applied to cultured neuronal networks with an air microscope objective (, 0.8 NA), P-DHM shows a coherent noise level typically corresponding to 1 nm at the single-pixel scale, in agreement with the -law, allowing to readily visualize the -wide thin neuronal processes with a signal-to-noise ratio of .Conclusions: Therefore, P-DHM represents a very promising label-free technique to study neuronal connectivity and its development, including neurite outgrowth, elongation, and branching.

Calibration of Near-Infrared Detectors Using a Wavelength Tunable Light Source

This paper presents the spectral responsivity calibrations of two indium gallium arsenide (InGaAs) and one germanium based near-infrared photovoltaic detectors using a wavelength tunable laser source based on a supercontinuum laser developed at the Metrology Research Institute, Aalto University. The setup consists of a supercontinuum laser based on a photonic crystal fiber as the light source, a laser line tunable filter, and coupling optics. These responsivity calibrations are performed against a pyroelectric radiometer over a wide spectral range of 800–2000 nm. Our wavelength tunable laser source has a high spectral power up to 2.5 mW with a narrow spectral full-width-at-half-maximum of 3 nm at a wavelength of 1100 nm. Despite the sharp spectral intensity variations, no artifacts are observed in the spectral responsivities of the detectors. Comparison of the spectral responsivities of the InGaAs detectors measured using the wavelength tunable laser and the earlier calibrations performed at the Metrology Research Institute in 2010 and 2016, shows that the higher spectral power of wavelength tunable light source decreases the expanded uncertainty from approximately 4% to 2.2–2.6% over the spectral range of 820–1600 nm. Temperature dependence of the spectral responsivities near the band gap edges are also measured and analysed.

Nonequilibrium hot-electron-induced wavelength-tunable incandescent-type light sources

The collective oscillation of electrons located in the conduction band of metal nanostructures being still energized, with the energy up to the bulk plasmon frequency, are called nonequilibrium hot electrons. It can lead to the state-filling effect in the energy band of the neighboring semiconductor. Here, we report on the incandescent-type light source composed of Au nanorods decorated with single Ga-doped ZnO microwire (AuNRs@ZnO:Ga MW). Benefiting from Au nanorods with controlled aspect ratio, wavelength-tunable incandescent-type lighting was achieved, with the dominating emission peaks tuning from visible to near-infrared spectral regions. The intrinsic mechanism was found that tunable nonequilibrium distribution of hot electrons in ZnO:Ga MW, injected from Au nanorods, can be responsible for the tuning emission features. Apart from the modification over the composition, bandgap engineering, doping level, etc., the realization of electrically driving the generation and injection of nonequilibrium hot electrons from single ZnO:Ga MW with Au nanostructure coating may provide a promising platform to construct electronics and optoelectronics devices, such as electric spasers and hot-carrier-induced tunneling diodes.

波長分割多重(WDM)通信用波長可変光源技術

波長分割多重(WDM)通信用波長可変光源技術

Mid-Infrared Spectral Compression of Soliton Pulse in an Adiabatically Suspended Silicon Waveguide Taper

Spectral compression (SPC) can be used for generating narrow bandwidth and wavelength-tunable light sources, which have important applications in optical communication system, spectroscopy, and nonlinear microscopy. In this paper, we numerically demonstrate the high-degree SPC of the chirp-free femtosecond pulse at wavelength 2.4 μm in a 6-cm long adiabatically suspended silicon waveguide taper. The silicon waveguide taper is designed with a dispersion-increasing profile along the propagation distance z . Simulation results show that the SPC factor can be up to 10.9, along with the brightness-enhanced factor of 8.0 and negligible sidelobe. The impacts of the higher order dispersion, higher order nonlinearity, losses (including linear and nonlinear loss), and variation of Kerr nonlinear coefficient along z on the SPC are also investigated. It is found that variation of Kerr nonlinear coefficient γ ( z ) and linear loss are the dominant perturbation to the degradation of the SPC performance.

Depth detection capability and ultra-large depth of field in imaging through a thin scattering layer

A wavelength-depth-matching method based on speckle correlation is proposed to achieve an ultra-large depth of field (DOF) of 90.5 mm experimentally on one side in imaging through a thin scattering layer. By making rational use of chromatic aberrations to compensate for the effects of depth differences, we determine the depth information of a target object using a incoherent wavelength-tunable light source. A random matrix that satisfies the distribution with a root mean square of 1 μm and a correlation length of 100 μm is generated to simulate the varying heights of the scattering surfaces which is used to demonstrate that larger DOF can be attained theoretically. This method, with its improved depth detection capability and ultra-large DOF, makes 3D dynamic imaging through scattering media possible.

High-Power and High-Order Random Raman Fiber Lasers

High-order random Raman fiber lasers are investigated for generating high-power widely wavelength-tunable light sources. By cascaded random Raman lasing and adjusting pump laser wavelength, continuous wavelength tuning from 1 to 1.9 μm is reported. For power scaling, a high-power Yb-doped fiber laser at 1064 nm with improved temporal stability is developed as the pump source. The ninth-order cascaded random Raman fiber laser with a spectral purity of 86.6% is achieved at 1806 nm. Up to 100.1-W inband power is obtained with an optical efficiency of 38.4% from 1064 nm and 27.2% from 915 nm. The results prove that cascaded random Raman fiber laser is a versatile technology to generate high-power fiber laser at wavelength outside the rare earth emission bands.

A stable wavelength-tunable triggered source of single photons and cascaded photon pairs at the telecom C-band

The implementation of fiber-based long-range quantum communication requires tunable sources of single photons at the telecom C-band. Stable and easy-to-implement wavelength-tunability of individual sources is crucial to (i) bring remote sources into resonance, (ii) define a wavelength standard, and (iii) ensure scalability to operate a quantum repeater. So far, the most promising sources for true, telecom single photons are semiconductor quantum dots, due to their ability to deterministically and reliably emit single and entangled photons. However, the required wavelength-tunability is hard to attain. Here, we show a stable wavelength-tunable quantum light source by integrating strain-released InAs quantum dots on piezoelectric substrates. We present triggered single-photon emission at 1.55 μm with a multi-photon emission probability as low as 0.097, as well as photon pair emission from the radiative biexciton–exciton cascade. We achieve a tuning range of 0.25 nm which will allow us to spectrally overlap remote quantum dots or tuning distant quantum dots into resonance with quantum memories. This opens up realistic avenues for the implementation of photonic quantum information processing applications at telecom wavelengths.

Quasi-phase-matched self-pumped optical parametric oscillation in a micro-resonator.

Lasing and self-pumped optical parametric oscillation (self-OPO) are achieved in a high-Q whispering-gallery-mode micro-resonator, made of neodymium-doped lithium niobate. A laser process providing 5 mW output power at 1.08 μm wavelength is sufficient to pump a self-OPO process within the same high-Q cavity. At 6 mW lasing output power, the sum of signal and idler output powers exceeds 1.2 mW. The wavelength of the generated light ranges from 1.5 to 3.8 μm. Phase matching is provided by a radial quasi-phase-matching structure, which is generated by a current-controlled calligraphic poling technique. To the best of our knowledge, this is the first demonstration of a quasi-phase-matched self-pumped nonlinear optical process in a micro-resonator, as well as the first self-OPO in a micro-resonator. The concept bears the potential for a highly integrated and wavelength-tunable coherent light source at low cost.

Control of light polarization by voltage in excitonic metasurface devices

We propose active metasurface devices where the state of emitted light is controlled by voltage. Based on the simulations of expected emission spectra, we present the concept of a light emitting device with the voltage controlled wavelength and degree of linear polarization of emission. The device combines the ability of metasurfaces to control light with a wavelength-tunable light source based on indirect excitons in coupled quantum well heterostructures.

Recent Advances and Applications of External Cavity-QCLs towards Hyperspectral Imaging for Standoff Detection and Real-Time Spectroscopic Sensing of Chemicals

External-cavity quantum cascade lasers (EC-QCL) are now established as versatile wavelength-tunable light sources for analytical spectroscopy in the mid-infrared (MIR) spectral range. We report on the realization of rapid broadband spectral tuning with kHz scan rates by combining a QCL chip with a broad gain spectrum and a resonantly driven micro-opto-electro-mechanical (MOEMS) scanner with an integrated diffraction grating in Littrow configuration. The capability for real-time spectroscopic sensing based on MOEMS EC-QCLs is demonstrated by transmission measurements performed on polystyrene reference absorber sheets, as well as on hazardous substances, such as explosives. Furthermore, different applications for the EC-QCL technology in spectroscopic sensing are presented. These include the fields of process analysis with on- or even inline capability and imaging backscattering spectroscopy for contactless identification of solid and liquid contaminations on surfaces. Recent progress in trace detection of explosives and related precursors in relevant environments as well as advances in food quality monitoring by discriminating fresh and mold contaminated peanuts based on their MIR backscattering spectrum is shown.

Size-effect of oligomeric cholesteric liquid-crystal microlenses on the optical specifications.

In cholesteric liquid-crystalline microlenses, we have studied the role of the microlens size on the focused light intensity and the focal length. We have found that the intensity is maximized by aiming a specific range for the diameter and the thickness of microlenses and that the focal length is adjusted by controlling the diameter and the annealing time of the optical film. Cholesteric microlenses may be used as wavelength-tunable directional light sources in organic soft-matter circuits.

Ultra-compact wavelength-tunable quantum-dot laser with silicon-photonics double ring filter

Compact, wavelength-tunable light sources are desired for the enhancement of information communication technology and bio-imaging applications. We propose a compact, wavelength-tunable laser diode with a wide wavelength-tunable range around 1230 nm consisting of a quantum-dot optical amplifier and a silicon photonic tunable filter. High-quality InAs quantum dots grown with the sandwiched sub-nano separator technique were used as the optical gain medium. The wavelength-tunable filter was constructed with ring resonators fabricated using silicon photonics. The single-mode laser oscillation was demonstrated with a 28.5-nm wavelength-tunable range.

Integrated Wavelength-Tunable Light Source for Optical Gas Sensing Systems

A compact instrument consisting of a distributed feedback laser (DFB) at 1.65 μm was developed as a light source for gas sensing systems using tunable diode laser absorption spectroscopy (TDLAS) technique. The wavelength of laser is tuned by adjusting the laser working temperature and injection current, which are performed by self-developed temperature controller and current modulator respectively. Stability test shows the fluctuation of the laser temperature is within the range of ±0.02°C. For gas detection experiments, the wavelength is tuned around the gas absorption line by adjusting laser temperature and is then shifted periodically to scan across the absorption line by the laser current modulator, which generates a 10 Hz saw wave signal. In addition, the current modulator is able to generate sine wave signal for gas sensing systems using wavelength modulation spectroscopy (WMS) technique involving extraction of harmonic signals. The spectrum test proves good stability that the spectrum was measured 6 times every 10 minutes at the constant temperature and current condition. This standalone instrument can be applied as a light source for detection systems of different gases by integrating lasers at corresponding wavelength.

Continuously Wavelength-Tunable Light Source With Constant-Power Output for Elimination of Residual Amplitude Modulation

A continuously wavelength-tunable light source with constant-power output is presented. The unwanted optical power modulation companying with the wavelength modulation of the distributed feedback laser diode by the injection current is successfully offset by the external amplitude modulation of electro-optic modulator. The optical power stability has been experimentally demonstrated better than 1.2% in a wavelength-tunable range of 0.876 cm -1 . The background baseline appearing in the direct absorption spectrum can be removed with the new light source. Not only that, the spectral distortion of the second harmonic signal caused by residual-amplitude modulation can also be largely eliminated. Experimental results have shown that the new light source is more suitable to be served as a pure wavelength-tuning reference source for different modes of spectroscopic measurement.

Speckle suppression in digital holographic imaging with random phases and different wavelengths

An effective speckle suppression method in digital holography is experimentally demonstrated with a wavelength-tunable coherent light source and a moving diffuser. Multiple off-axis digital holograms with different wavelengths and random phases are obtained. The Fresnel transformation algorithm is used to calculate the diffraction integral in the reconstruction. The speckle noise in the reconstructed images is suppressed by super- posing and averaging the intensity of multiple reconstruction images. Experiment results and the parameter evaluation show that the presented method is effective and feasible. © 2014 Society of Photo-Optical Instrumentation Here, a novel method for generating multiple holograms and suppressing speckle is presented. A wavelength-tunable coherent light source (LS) and a moving diffuser are used to generate multiple holograms with random phases (RP) and DWs. The intensity superposition and average of recon- structed images are performed to suppress the speckle noise. The DW intervals are experimentally used to generate holograms. The presented method of speckle suppression is compared with the random-phase intensity superposition and DWs in the experiment. The performance parameters are cal- culated and compared to demonstrate the effect of the speckle suppression.