Wavelength-tunable Laser Source Research Articles

Widefield Super-Resolution Infrared Spectroscopy and Imaging of Autofluorescent Biological Materials and Photosynthetic Microorganisms Using Fluorescence Detected Photothermal Infrared (FL-PTIR).

We have demonstrated high-speed, super-resolution infrared (IR) spectroscopy and chemical imaging of autofluorescent biomaterials and organisms using camera-based widefield photothermal detection that takes advantage of temperature-dependent modulations of autofluorescent emission. A variety of biological materials and photosynthetic organisms exhibit strong autofluorescence emission under ultraviolet excitation and the autofluorescent emission has a very strong temperature dependence, of order 1%/K. Illuminating a sample with pulses of IR light from a wavelength-tunable laser source causes periodic localized sample temperature increases that result in a corresponding transient decrease in autofluorescent emission. A low-cost light-emitting diode-based fluorescence excitation source was used in combination with a conventional fluorescence microscopy camera to detect localized variations in autofluorescent emission over a wide area as an indicator of localized IR absorption. IR absorption image stacks were acquired over a range of IR wavelengths, including the fingerprint spectral range, enabling extraction of localized IR absorption spectra. We have applied widefield fluorescence detected photothermal IR (FL-PTIR) to an analysis of autofluorescent biological materials including collagen, leaf tissue, and photosynthetic organisms including diatoms and green microalgae cells. We have also demonstrated the FL-PTIR on live microalgae in water, demonstrating the potential for label-free dynamic chemical imaging of autofluorescent cells.

3D nanopolymerization and damage threshold dependence on laser wavelength and pulse duration.

The dependence of the polymerization and optical damage thresholds in multi-photon polymerization (MPP) lithography was studied using a broadly-tunable laser system with group delay dispersion (GDD) control. The order of non-linearity and the light-matter interaction mechanisms were investigated using the resolution bridges method for non-photosensitized SZ2080TM and photosensitized SZ2080TM + IRG369 prepolymers. Energy deposition, voxel dimension growth, and the size of the dynamic fabrication window (DFW) were measured in the 700-1300nm wavelength range at three different pulse durations measured at the sample - 100, 200 and 300fs. Polymerization was observed at all wavelengths and pulse durations without significant differences in the achieved minimal spatial dimension ( nm). This was achieved despite the broad range of excitation wavelengths used which spanned two- and three-photon absorption bands, and the differences in the absorption spectra of the prepolymers. The lateral and longitudinal voxel growth dynamics revealed an abrupt change in the power dependence of polymerization and a significant variation of the DFW - from 1 at 1250nm to 29 at 700nm. This result can be interpreted as a consequence of a change in the instantaneous refractive index and a lowering of the polymerization but not the damage threshold. The optimization of energy delivery to the material by a wavelength-tunable laser source with pulse duration control was experimentally validated. These findings are uncovering the complexity of polymerization mechanisms and are useful in further development of MPP technology.

Nanomechanical Spectroscopy of 2D Materials.

We introduce a nanomechanical platform for fast and sensitive measurements of the spectrally resolved optical dielectric function of 2D materials. At the heart of our approach is a suspended 2D material integrated into a high Q silicon nitride nanomechanical resonator illuminated by a wavelength-tunable laser source. From the heating-related frequency shift of the resonator as well as its optical reflection measured as a function of photon energy, we obtain the real and imaginary parts of the dielectric function. Our measurements are unaffected by substrate-related screening and do not require any assumptions on the underling optical constants. This fast (τrise ∼ 135 ns), sensitive (noise-equivalent power = ), and broadband (1.2-3.1 eV, extendable to UV-THz) method provides an attractive alternative to spectroscopic or ellipsometric characterization techniques.

Waveguide scattering antennas made by direct laser writing in bulk glass for spectrometry applications in the short-wave IR.

A buried straight waveguide perturbed periodically by six antennas composed of submicronic cylinder voids is entirely fabricated using ultrafast laser photoinscription. The light scattered from each antenna is oriented vertically and is detected by a short-wave IR camera bonded to the surface of the glass with no relay optics. The response of each antenna is analyzed using a wavelength tunable laser source and compared to simulated responses verifying the behavior of the antenna. These results show the good potential of the direct laser writing technique to realize monolithic embedded detectors by combining complex optical functions within a 3D design. A wavelength meter application with a spectral resolution of 150 pm is proposed to demonstrate this combination.

Broadband two-dimensional spectrochronography with ultrashort pulses in the mid-infrared

A novel, 2.6-to-10-μm wavelength-tunable laser source of sub-70-fs pulses is combined with mid-IR heterodyne detection to provide a versatile laser platform for broadband two-dimensional Fourier-transform infrared spectrochronography. The mid-infrared laser output serves as both a short-pulse driver and a broadband probe for time-resolved studies of ultrafast molecular coherence, population dynamics, and multimodal energy transfer in a vast class of complex molecular systems.

Low-Cost Interrogation of Long-Distance and Multipoint FBG Sensor Using Incoherent-FMCW Optical Ranging System

More than a decade, multipoint sensor systems involving fiber Bragg gratings (FBGs) have been widely used in various implementations, especially the structural health monitoring system (SHMS). In this paper, a three-points FBG sensor was successfully analyzed by combination of incoherent frequency modulated continuous wave (I-FMCW) optical ranging system and a vertical-cavity surface-emitting laser (VCSEL) as a wavelength-tunable laser source. This system is not only capable of reading out the FBG sensors, but also identifying the location of the installed FBGs. The I-FMCW can be conducted by intensity modulation of a low-cost commercial VCSEL. Furthermore, the wavelength of the VCSEL can also be tuned by the injection current change to match the Bragg wavelength of FBGs. The detected signal from the FBGs and the reference signal from the signal generator are electrically mixed to obtain the beat frequency. A thermo-electric controller (TEC) controls temperature of one of the FBGs in the range of 25° C ~ 45° C, and contrary the others are kept at room temperature. As well as measuring temperature, one of the FBGs is strained in the range of $369~\mu \varepsilon \sim ~2137~\mu \varepsilon $ . The measurement result shows that the system can clearly distinguish all FBGs installed in totally 6.6 km-long optical fiber. In addition, the Bragg wavelength shift of the FBGs according to temperature as well as strain changes was successfully measured. The system can be scaled up more than three FBGs and extended scale area, and is a candidate as a low-cost and simple FBG interrogator for a long-range SHMS.

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.

An Erbium-Doped Fiber Whispering-Gallery-Mode Microcavity Laser

A kind of whispering-gallery-mode (WGM) microcavities has been fabricated by melting a corroded erbium-doped fiber with a fiber-fusion splicer. The quality factor (Q) value of the microcavity can reach as high as $3\times 10^{7}$ . The microcavity is pumped by a wavelength-tunable laser source, and due to the high-Q characteristics, the laser threshold can be as low as 21.2 $\mu \text{W}$ in a 60- $\mu \text{m}$ -diameter microsphere-like cavity. By tuning the frequency of the pump laser at 1485 nm with 19.6 GHz, we can tune the WGM microcavity laser by 17.3 GHz around 1578.25 nm.

A Method for Measurement of Nonlinearity of Laser Interferometer Based on Optical Frequency Tuning.

A method for measuring the nonlinearity of laser interferometer using optical frequency tuning technique is presented in this paper. The basic principle of this method is to make the fractional part of an interference fringe change by tuning the laser frequency and determining the nonlinearity of interferometer by comparing the fractional fringe change measured by the interferometer to that calculated from the laser frequency change. An experimental interferometric system with a wavelength tunable laser source is set up and the nonlinearity of the interferometer is measured. Since it does not require the precise displacement mechanism to produce the optical path difference change, this method is more convenient to use and may achieve a higher accuracy than the conventional measurement methods. The nonlinearity of the arbitrary interferometric phase can be measured by changing the laser frequency with this method. Experiments results have shown that the repeatability of nonlinearity measurement is less than 0.2 nm. This method can be applied to interferometry-based high precision dimensional measurements, such as coordinate measurement and displacement sensor calibration.

High-Power Narrow-Linewidth Tunable 670.8-nm Master Oscillator Power Amplifier With High Efficiency

A highly efficient 670.8-nm high-power narrow-linewidth wavelength-tunable laser source with master oscillator power amplifier structure is demonstrated, which yielded CW output power of 4.5 W with spectral linewidth of 0.3 pm and mode-hop free tuning range of 52 pm (35 GHz). The total conversion efficiency of 20% was achieved. The developed narrow-linewidth tunable laser source can provide better performance than others ever reported at 670.8 nm for many applications, such as laser isotope separation, Bose–Einstein condensation experiments, and mid-infrared laser generation.

Wide-dynamic-range cantilever magnetometry using a fiber-optic interferometer and its application to high-frequency electron spin resonance spectroscopy

We present a method of broadening the dynamic range of optical interferometric detection of cantilever displacement. The key idea of this method is the use of a wavelength-tunable laser source. The wavelength is subject to proportional-integral control, which is used to keep the cavity detuning constant. Under this control, the change in wavelength is proportional to the cantilever displacement. Using this technique, we can measure large displacements (>1 µm) without degrading the sensitivity. We apply this technique to high-frequency electron spin resonance spectroscopy and succeed in removing an irregular background signal that arises from the constantly varying sensitivity of the interferometer.

2.5–10 Gbit/s laser source based on two optical-injection Fabry–Perot laser diodes

In this investigation, a wavelength tunable laser source based on optical-injection of two Fabry–Perot laser diodes (FP-LDs) is demonstrated. In the proposed laser source, a self-injection locked master FP-LD provides a stable continuous-wave (CW) lasing, which wavelength can be selected by means of an optical tunable bandpass filter (TBF). The CW wavelength is launched into another FP-LD (the slave laser) for direct signal modulation. In this two optical-injection FP-LD architecture, directly modulation of 2.5Gbit/s on–off keying (OOK) and 10Gbit/s orthogonal frequency division multiplexing (OFDM) signals can be achieved in the proposed laser source with negligible power penalty after 25km single-mode-fiber (SMF) transmission.

High-resolution optical spectrum characterization using optical channel estimation and spectrum stitching technique

A technique is proposed to measure the high-resolution and wide-band characterization of amplitude, phase responses, and polarization property of optical components. This technique combines the optical spectrum stitching and optical channel estimation methods. Two kinds of fiber Bragg grating based Fabry-Perot cavities with ultrafine structures have been characterized based on this technique. By using 1024 point fast Fourier transform and a narrow linewidth, wavelength-tunable laser source, a frequency resolution of ~10 MHz is realized with an optical measurement range beyond 250 GHz.

Continuously Wavelength-Tunable Laser Source Using a Self-Injected Fabry–Pérot Laser Diode

In this letter, a continuous-wave (CW) continuously wavelength-tunable laser source employing a self-injected Fabry-Pérot laser diode (FP-LD) is achieved and investigated experimentally. Through carefully tuning the fiber Fabry-Pérot tunable filter (FFP-TF) and chip temperature of the FP-LD, 80 C-band International Telecommunication Union (ITU) wavelengths with 50-GHz mode spacing were obtained with a good performance of the output peak power >;-4.74 dBm (ΔP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> = 2.40 dB) and optical sidemode suppression ratio (SMSR) >;41 dB.

Extended-Cavity Semiconductor Wavelength-Swept Laser for Biomedical Imaging

We demonstrate a compact high-power rapidly swept wavelength tunable laser source based on a semiconductor optical amplifier and an extended-cavity grating filter. The laser produces excellent output characteristics for biomedical imaging, exhibiting >4-mW average output power, <0.06-nm instantaneous linewidth, and >80-dB noise extinction with its center wavelength swept over 100 nm at 1310 nm at variable repetition rates up to 500 Hz.

Characterization of filter radiometers with a wavelength-tunable laser source

Results of filter radiometer characterization with a wavelength-tunable Ti : sapphire laser in the wavelength band around 900 nm are presented. The effect of interference between the reflections from filter surfaces in the case of coherent laser light was studied and reduced with a special filter design with antireflection coatings. Measuring the responsivity as a function of wavelength over a very narrow band was used to reveal the remaining interference effects. Uncertainty analysis and test results indicate that filter radiometers can be characterized with a relative standard uncertainty of 9×10−4 using the scanning method. The results agree well with more conventional monochromator-based measurements.