Low-coherence Light Source Research Articles

Low-Coherence Shearing Interferometry With Constant Off-Axis Angle

We present a wide-field interferometric imaging module for biomedical and metrological measurements, employing shearing interferometry with constant off-axis angle (SICA) that can work, for the first time, with a low-coherence light source. In the SICA module, the shearing distance between the interfering beams can be fully controlled without a direct relation with the off-axis angle. In contrast to our previous SICA module, here we use a low-coherence illumination source, providing quantitative phase profiles with significantly lower spatial coherent noise. Although a low-coherence source is used, we obtain off-axis interference on the entire camera sensor, where the optical path difference between the two beams is compensated by using a glass window positioned in the confocal plane. This highly stable, common-path, low-coherence, single-shot interferometric module can be used as an add-on unit to a conventional bright-field microscope illuminated by a low-coherence source. We demonstrate the advantages of using the module by quantitative phase imaging of a polymer bead, fluctuations in a human white blood cell, and dynamics of human sperm cells.

Low-coherence optical diffraction tomography using a ferroelectric liquid crystal spatial light modulator.

Optical diffraction tomography (ODT) is a three-dimensional (3D) label-free imaging technique. The 3D refractive index distribution of a sample can be reconstructed from multiple two-dimensional optical field images via ODT. Herein, we introduce a temporally low-coherence ODT technique using a ferroelectric liquid crystal spatial light modulator (FLC SLM). The fast binary-phase modulation provided by the FLC SLM ensures the high spatiotemporal resolution. To reduce coherent noise, a superluminescent light-emitting diode is used as an economic low-coherence light source. We demonstrate the performance of the proposed system using various samples, including colloidal microspheres and live epithelial cells.

A broadband Faraday fiber-optic current sensor with excess noise compensation

A Faraday fiber-optic current sensor with suppression of excess noise produced by a low-coherent light source is presented. The sensor is designed to detect short current pulses and represents an in-line interferometer with a sensing element based on a spun highly birefringent fiber. To suppress the excess noise, a two-channel light detection scheme without an additional delay line is applied. It is established that bending birefringence induced in the spun fiber wound on a coil limits the noise suppression. To minimize this effect, a new geometry of the sensing coil is used. The proposed sensor exhibits a 5-fold improvement of the signal-to-noise ratio and the same enhancement of the sensitivity to a value of 0.8A in the 3.5 MHz bandwidth. The minimum duration of the measured current pulse is 200 ns.

Time-domain optical coherence tomography and gelatin-based skin phantom as training tools for venipuncture

Optical coherence tomography (OCT) is a non-invasive imaging modality developed in the early 1990’s for retinal imaging. Further modifications allowed OCT’s to be used on other parts of the human body, and non-medical areas as well. Time-domain OCT (TD-OCT) basically is a Michelson interferometer with a low-coherence light source which enables non-invasive, cross-sectional visualization of the sample. In this paper, the researchers used the TD-OCT system, which uses a rotating reference mirror. Using this system in conjunction with fabricated skin phantoms, students of venipuncture can hone their skills on skin phantoms before practicing on actual persons. This helps in mitigating the potential risks inherent to the performance of invasive procedures such as venipuncture. Furthermore, the skin phantom is chiefly composed of gelatin with latex tubing used to imitate the veins. Such materials are cheap and readily available, allowing multiple phantoms to be fabricated easily. Preliminary results showed that OCT is a promising tool in imaging the tissue phantom and thus can be utilized for venipuncture training.

Metal-embedded fiber optic sensor packaging and signal demodulation scheme towards high-frequency dynamic measurements in harsh environments

Optical fiber-based sensors are commonly used for industrial monitoring and control processes due to their small size, resistance to electromagnetic interference, and ability to perform a wide variety of high-precision measurements. However, implementing optical fiber sensors in harsh environments is challenging because they are small and fragile. Proper packaging of fiber-optic sensors could extend their use to harsh environments, including at high temperature and under high radiation. Furthermore, conventional fiber optic-based measurement systems often use computationally expensive signal processing algorithms which hinder their use in high-frequency dynamic applications. This work reports on the design of an optical fiber-pressure sensor system based on low-coherence interferometry that uses a metal-embedded optical fiber to provide a robust sensor package. A novel phase demodulation scheme is proposed to extract length changes from the deformation of a thin diaphragm within a Fabry–Pérot cavity to measure pressure. The sensor has been tested to ±100kPa, has a theoretical linear response over a ±270kPa dynamic range (24dB maximum signal-to-noise ratio), and can resolve pressure transients up to 3910kPa/s. Sampling at 100kHz, the present sensor can resolve 2kPa dynamic pressures at frequencies up to 2kHz. Faster transients on the order of tens to hundreds of kHz can theoretically be resolved at the expense of decreasing the maximum resolvable amplitude. A methodology for designing a LCI pressure sensor for a given application is outlined based on the limitations imposed by the Nyquist criterion, the diaphragm's resonant frequency, the LCI optoelectronics, and the phase demodulation scheme. The sensor is the first to implement a low-coherence light source and a Fabry–Pérot interferometer designed to provide real-time high-fidelity pressure measurements using a metal-embedded optical fiber. The demonstrated sensor provides a platform for sensing in harsh conditions such as in nuclear and energy applications.

High-resolution nonfluorescent imaging with structured illumination for patterned surface measurement

Surface patterning is widely performed in various applications, and hence high-resolution, high-throughput, and nondestructive measurement of patterned surfaces is required. The current optical methods used for such measurements are constrained, in terms of spatial resolution, by the diffraction limit. In this study, structured illumination was employed for high-resolution wide-field imaging measurements. Structured illumination microscopy is well known as a fluorescent bio-imaging technique. For nonfluorescent imaging and industrial measurement applications, however, speckle noise, generated by the structured illumination pattern, environmental vibrations, and thermal drift, causes problems for the high-precision positioning and control of the illumination pattern. To solve these problems, we developed a structured illumination microscope that detects the phase of the structured illumination as a fringe signal by using an additional interferometric measurement system. In this system, a low-coherence light source is employed for illumination pattern generation and high-resolution imaging with low speckle noise. The constructed optical system consists of multiple Michelson interferometers that allow the manipulation of low-coherence interference fringes at the sample position and the detection of the phase via another sensor. Robust imaging can be implemented by feeding back the structured illumination phase through super-resolution image processing. We experimentally achieved high-resolution nonfluorescent imaging with low speckle noise and resolved patterned surfaces beyond the diffraction limit without high-precision positioning stages. Furthermore, the image reconstruction algorithm typically used in structured illumination microscopy was improved by adopting a Fourier ptychography approach. Thus, artifact reduction, as well as decreases in luminance and pitch errors, was achieved, and this result represents a step forward in the development of practical measurement methods for patterned surfaces.

Fabrication and characterization of femtosecond laser induced microwave frequency photonic fiber grating.

We proposed and fabricated a microwave-frequency photonic fiber grating (MPFG) by femtosecond laser micromachining on optical fibers. Illuminated by low coherent light source, the MPFG can be interrogated using proposed microwave photonic system to show the resonant peaks in microwave frequency domain. We studied the working principle and characteristics of this device. After that, we discussed the influence of fiber type, apodization and light source coherence lengths on this device. The device can also respond to ambient temperature change like fiber optic sensors.

Sampling moiré method: a tool for sensing quadratic phase distortion and its correction for accurate quantitative phase microscopy.

The advantages of quantitative phase microscopy (QPM) such as label-free imaging with high spatial sensitivity, live cell compatibility and high-speed imaging makes it viable for various biological applications. The measurement accuracy of QPM strongly relies on the shape of the recorded interferograms, whether straight or curved fringes are recorded during the data acquisition. Moreover, for a single shot phase recovery high fringe density is required. The wavefront curvature for the high-density fringes over the entire field of view is difficult to be discerned with the naked eye. As a consequence, there is a quadratic phase aberration in the recovered phase images due to curvature mismatch. In the present work, we have implemented sampling moiré method for real-time sensing of the wavefront curvature mismatch between the object and the reference wavefronts and further for its correction. By zooming out the interferogram, moiré fringes are generated which helps to easily identify the curvature of the fringes. The wavefront curvature mismatch correction accuracy of the method is tested with the help of low temporal coherent light source such as a white light (temporal coherence ∼ 1.6 µm). The proposed scheme is successfully demonstrated to remove the quadratic phase aberration caused due to wavefront mismatch from an USAF resolution target and the biological tissue samples. The phase recovery accuracy of the current scheme is further compared with and found to better than the standard method called principle component analysis. The proposed method enables recording of the corrected wavefront interferogram without needing any additional optical components or modification and also does not need any post-processing correction algorithms. The proposed method of curvature compensation paves the path for a high-throughput and accurate quantitative phase imaging.

Multi-wavelength coherent random laser in bio-microfibers.

In this paper, pure silk protein was extracted from Bombyx mori silks and fabricated into a new kind of disordered bio-microfiber structure using electrospinning technology. Coherent random lasing emission with low threshold was achieved in the silk fibroin fibers. The random lasing emission wavelength can be tuned in the range of 33 nm by controlling the pump location with different scattering strengths. Therefore, the bio-microfiber random lasers can be a wide spectral light source when the system is doped with a gain or energy transfer medium with a large fluorescence emission band. Application of the random lasers of the bio-microfibers as a low-coherence light source in speckle-free imaging had also been studied.

Size measurement of Daphnia pulex using low-coherence Gabor digital holography

We measured the size of Daphnia pulex (D. pulex), which is an indicator organism used for examining toxicity, under freely-swimming and stress-free conditions, made possible by the post-focusing ability of digital holography (DH). This environmental measurement is a new application of DH. This implementation, which is a key point of this research, should be applicable to real on-site use in non-sterile and unstable mechanical situations. Through trial and error, we arrived at the simplest interferometer configuration using a single beam line that can withstand vibrations, a low-coherence light source for reducing coherent noise, and single-shot image capturing. Automatic size measurement based on DH with image processing was performed. The originality of this research is the quantitative size measurement and performance estimation achieved by comparing the results with those from conventional manual eye measurement using ordinary optical microscope observation and image processing software. The estimation also allowed us to determine the proper parameters in the image processing for DH measurement.

Wavefront sensing for low-coherent sources through spatial coherence revival

An interferometry based wavefront sensing scheme applicable to low-coherent light sources is proposed and demonstrated. Using a pair of cascaded Sagnac interferometers, spatial frequency multiplexed interference of three sheared copies of the wavefront is recorded. The independent vector gradients obtained are used to reconstruct the wavefront. We overcome the spatio-temporal coherence limitations of the light source through the choice of spatial and spectral distribution of the light source. A Gaussian spectral distribution enables the measurement of the phase gradient corresponding to the central wavelength. Introduction of aperture for controlling the spatial distribution of the extended light source, we revive the spatial coherence of the optical field at the detector plane. We revive the spatial coherence of the field for the set value of lateral shear to maintain the visibility of the interference at the detectable level. Validating the scheme through measurement of changes in wavefront as a function of a collimation lens position, we apply it to reconstruct the wavefront of a light beam passing through a pseudo random phase plate, an atmospheric turbulence simulator.

Conical refraction with low-coherence light sources.

We report on conical refraction (CR) with low-coherence light sources, such as light-emitting diodes and decoherentized HeNe laser radiation, and demonstrate different CR patterns. In our experiments, a variation of the pinhole sizes from 25 to 100 µm and the distances to pinhole from 50 to 5 cm reduced spatial coherence of radiation that resulted in the disappearance of the dark Poggendorff's ring in the Lloyd's plane. This is attributed to the interference nature of the Lloyd's distribution and found to be in excellent agreement with the paraxial dual-cone model of conical refraction.

Multimode Random Fiber Laser for Speckle-Free Imaging

Light sources with high radiance are increasingly required for full-field real-time imaging. Conventional lasing sources are poorly suited for such imaging due to their high spatial or temporal coherence, which generates a speckle that deteriorates image quality. Here, a random fiber laser with multitransverse modes is used as an illumination light source to effectively reduce the speckle in imaging. Low spatial coherence and low temporal coherence of the random fiber laser give birth to significant reduction in the speckle. Under the power-limited condition, the multimode random fiber laser is verified to have a comparable or even better imaging quality compared to a multimode amplified spontaneous emission source. Furthermore, its potential to generate ultrahigh power of up to hundreds of Watts with extremely-high spectral density would make a breakthrough in the development of a new generation of high-power low-coherence light sources for many speckle-free imaging applications, where conventional light sources are not usable. As the multimode random fiber laser can naturally inherit all the advantages of single-mode random fiber lasers, including flexible wavelength, robust structure, and high power, this paper may provide a platform to develop powerful low-coherence light sources to meet wide range requirements of the full-field real-time speckle-free imaging.

Wavelength-multiplexed digital holography for quantitative phase measurement using quantum dot film.

We propose an enhanced quantitative three-dimensional measurement system using wavelength-multiplexed digital holography. To simplify the configuration, a dual-peak quantum dot wavelength converter, combined with a blue LED, is adapted as a single low-coherence light source. Rather than a conventional dual-wavelength method, which records and reconstruct the object wave for each wavelength, the proposed system can capture the holograms of two wavelengths simultaneously with fewer acquisitions, simple setup, and low noise. To verify the system's performance, the measurements of the step height sample are presented.

(Invited) Fine Temperature Monitoring of Si Wafer for Plasma Processing

One of the critical parameters in many semiconductor-processing steps such as plasma etching, chemical vapor deposition, and so on, is the substrate temperature. For example, the etch rate and etch profile are sensitive to the substrate temperature in the plasma etching process. However, contact measurement such as thermo-couples and fluorescent temperature sensor cannot measure the accurate temperature, especially in low pressure due to poor thermal transfer between the wafer and the probe. We have developed the non-contact temperature measurement of substrates by using optical low-coherence interferometer. [1-6] The low-coherence interferometry is based on Michelson interferometry, and a white light such as a super-luminescent diode (SLD) was used as a low-coherence light source. SLD has the central wavelengths of 1325 nm and the full width at half maximum of 64.5 nm. The light which is reflected at the surface of Si substrate interfered with the light reflected at the scanning reference mirror. The interfered light was detected with a spectrometer and the signal which is a frequency domain spectrum was converted to a time domain signal by inverse Fourier transformation. The temperature of the Si substrate is derived from the change in the optical path length of Si substrate due to thermal expansion and changes in the refractive index. The temperatures of Si wafer and chamber parts made of Si were successfully monitored in the SiO2 plasma etching using capacitively coupled plasma. The performance was compared with that of fluorescent temperature sensor. From these results, we have demonstrated highly precise and rapid response measurement of the temperature of Si wafer at real time by using Fourier domain low coherence interferometer.

Deep imaging in highly scattering media by combining reflection matrix measurement with Bessel-like beam based optical coherence tomography.

Multiple scattering in biomedical tissue limits the imaging depth within a range of 1-2 mm for conventional optical imaging techniques. To extend the imaging depth into the scattering medium, a computational method based on the reflection matrix measurement has been developed to retrieve the singly back-scattered signal light from the dominant detrimental multiple-scattered background. After applying singular value decomposition on the measured matrix in the post-process, the target image underneath the turbid media is clearly recovered. To increase the depth of focus of the incident light by elongating the focal spot along the optical axis, a digital grating pattern is specially designed and displayed on a phase-only spatial light modulator to generate the Bessel-like beam for lateral point scanning. According to the results, the depth of focus is increased up to 2.4 mm which is much longer than the value of ∼50 μm obtained by using the conventional focused Gaussian beam, leading to a deeper penetration depth due to the self-healing feature of the Bessel-like beam. In addition, generation of the Bessel-like beam simplifies the axial scanning process by getting rid of the need to mechanically translate the focal zone along the optical axis of an objective with a high numerical aperture. By combining this method with an optical coherence tomography system with a low coherence light source, a depth-resolved optical image is obtained underneath a highly turbid medium.

Virtual phase conjugation based optical tomography for single-shot three-dimensional imaging.

We propose a virtual phase conjugation (VPC) based optical tomography (VPC-OT) for realizing single-shot optical tomographic imaging systems. Using a computer-based numerical beam propagation, the VPC combines pre-modulation and post-demodulation of the probe beam's wavefront, which provides an optical sectioning capability for resolving the depth coordinates. In VPC-OT, the physical optical microscope system and VPC are coupled using digital holography. Therefore, in contrast to conventional optical tomographic imaging (OTI) systems, this method does not require additional elements such as low-coherence light sources or confocal pinholes. It is challenging to obtain single-shot three-dimensional (3D) tomographic images using a conventional OTI system; however, this can be achieved using VPC-OT, which employs both digital holography and computer based numerical beam propagation. In addition, taking into account that VPC-OT is based on a complex amplitude detection using digital holography, this method allows us to simultaneously obtain quantitative phase contrast images. Using an objective lens with a numerical aperture (NA) of 0.8, we demonstrate a single-shot 3D imaging of frog blood cells with a depth resolution of 0.94 μm.

Near-infrared and mid-infrared semiconductor broadband light emitters

Semiconductor broadband light emitters have emerged as ideal and vital light sources for a range of biomedical sensing/imaging applications, especially for optical coherence tomography systems. Although near-infrared broadband light emitters have found increasingly wide utilization in these imaging applications, the requirement to simultaneously achieve both a high spectral bandwidth and output power is still challenging for such devices. Owing to the relatively weak amplified spontaneous emission, as a consequence of the very short non-radiative carrier lifetime of the inter-subband transitions in quantum cascade structures, it is even more challenging to obtain desirable mid-infrared broadband light emitters. There have been great efforts in the past 20 years to pursue high-efficiency broadband optical gain and very low reflectivity in waveguide structures, which are two key factors determining the performance of broadband light emitters. Here we describe the realization of a high continuous wave light power of >20 mW and broadband width of >130 nm with near-infrared broadband light emitters and the first mid-infrared broadband light emitters operating under continuous wave mode at room temperature by employing a modulation p-doped InGaAs/GaAs quantum dot active region with a ‘J’-shape ridge waveguide structure and a quantum cascade active region with a dual-end analogous monolithic integrated tapered waveguide structure, respectively. This work is of great importance to improve the performance of existing near-infrared optical coherence tomography systems and describes a major advance toward reliable and cost-effective mid-infrared imaging and sensing systems, which do not presently exist due to the lack of appropriate low-coherence mid-infrared semiconductor broadband light sources.

Dual-wavelength digital holography with a low-coherence light source based on a quantum dot film.

This Letter proposes a dual-wavelength, low-coherence digital holography system with a single light source, which utilizes a quantum dot (QD) film as a wavelength converter. By changing the size of the QDs, the proposed method easily yields higher and more uniform illumination of any target wavelength, compared with bandpass-filtered light-emitting diodes. Fabrication parameters of the QD film for better conversion efficiency are discussed. Using this light source with the dual-wavelength reconstruction method extends the efficiency and range of nanoscale three-dimensional height measurements.

An overview of methods to mitigate artifacts in optical coherence tomography imaging of the skin.

Optical coherence tomography (OCT) of skin delivers three-dimensional images of tissue microstructures. Although OCT imaging offers a promising high-resolution modality, OCT images suffer from some artifacts that lead to misinterpretation of tissue structures. Therefore, an overview of methods to mitigate artifacts in OCT imaging of the skin is of paramount importance. Speckle, intensity decay, and blurring are three major artifacts in OCT images. Speckle is due to the low coherent light source used in the configuration of OCT. Intensity decay is a deterioration of light with respect to depth, and blurring is the consequence of deficiencies of optical components. Two speckle reduction methods (one based on artificial neural network and one based on spatial compounding), an attenuation compensation algorithm (based on Beer-Lambert law) and a deblurring procedure (using deconvolution), are described. Moreover, optical properties extraction algorithm based on extended Huygens-Fresnel (EHF) principle to obtain some additional information from OCT images are discussed. In this short overview, we summarize some of the image enhancement algorithms for OCT images which address the abovementioned artifacts. The results showed a significant improvement in the visibility of the clinically relevant features in the images. The quality improvement was evaluated using several numerical assessment measures. Clinical dermatologists benefit from using these image enhancement algorithms to improve OCT diagnosis and essentially function as a noninvasive optical biopsy.