Technical improvements applied to a double-pass setup for performance and cost optimization

A non-contact and non-invasion method to measure lens curvature radius was proposed. In the experiment, we developed a homemade Line Field Spectral Domain Optical Coherence Tomography (LF-OCT) System to measure lens curvature and focal length, using a broadband super luminescent diode (SLD) light source having a center wavelength of 830 nm. By using the methods of spectrum correction, the frequency, phase and amplitude can be corrected automatically and we can reach a higher accuracy. The result demonstrated that the LF-OCT has high accuracy and reliability and therefore it could be recommended to use in optical lens measurement due to its non-contact and non-invasion in nature.

Optical coherence tomography (OCT) is an emerging technology for high-resolution cross-sectional imaging of 3D structures. Not only could OCT extract the internal features of an object, but it could acquire the 3D profile of an object as well. Hence it has huge potentials for industrial applications. Owing to non-scanning along the X-Y axis, full-field OCT could be the simplest and most economic imaging system, especially for applications where the speed is critical. For an OCT system, the performance and cost basically depends on the light source being used. The broader the source bandwidth, the finer of the depth resolution that could be reached; the more power of the source, the better signal-to-noise ratio and the deeper of penetration the system achieves. A typical SLD (Superluminescent Diode) light source has a bandwidth of 15 nm and 10 mW optical power at a price around $6,000. However, a Halogen bulb having 50W power and 200nm bandwidth only costs less than $10. The design and implementation of a large-area, full-field OCT system using Halogen white-light source is described in the paper. The experimental results obtained from 3D shaping and multiple-layer tomographies are also presented.

Light synthesis with linearly chirped fibre Bragg gratings (FBGs) for optical coherence tomography (OCT) applications

This paper dedicated to the development of a surface profile measurement system based on the SLD (Super Luminescent Diode) light source for silicon wafer. The silicon wafer is very important as a substrate of semiconductor products such as integrated circuit (IC) chips, light emitting diodes (LEDs), solar cells and MEMS devices ^[1]. In this paper, Michelson interferometer is used to generate double beams in order to achieve interference. According to the acquired interference fringe image, we obtain the surface profile of the tested silicon wafer. As a light source, SLD has good spatial coherence and due to its higher transmittance to silicon wafers than other wavelength sources, high fringe visibility can be achieved. It can also perform a full scan in a larger step and achieve rapid on-line measurement of the target surface.

In this work we propose an open-top like common-path intrinsically achromatic optical diffraction tomography system. It operates as a total-shear interferometer and employs Ronchi-type amplitude diffraction grating, positioned in between the camera and the tube lens without an additional 4f system, generating three-beam interferograms with achromatic second harmonic. Such configuration makes the proposed system low cost, compact and immune to vibrations. We present the results of the measurements of 3D-printed cell phantom using laser diode (coherent) and superluminescent diode (partially coherent) light sources. Broadband light sources can be naturally employed without the need for any cumbersome compensation because of the intrinsic achromaticity of the interferometric recording (holograms generated by -1st and +1st conjugated diffraction orders are not affected by the illumination wavelength). The results show that the decreased coherence offers much reduced coherent noise and higher fidelity tomographic reconstruction especially when applied nonnegativity constraint regularization procedure.

A superluminescent diode (SLD) has properties, based on the degree of coherence, that are bounded by those of the light emitting diode and the laser diode. The SLD can be designed to meet a wide range of optical system needs. By introducing ridge-waveguide lateral confinement, a good anti-reflection coating at one end and a high reflectivity mirror at the other, we have demonstrated an SLD that allows 30 percent coupling efficiency into a lensed 0.23 NA, 50 μm diameter graded index core fiber. The power in the fiber is 550 μW at 250 mA and 20°C. It is possible to maintain a constant power level in the fiber greater than 250 μW over the temperature range 0 to 35°C by adjusting the current. The spectral width is 300 A and the modulation bandwidth 350 MHz. PCM with 400 Mbit/s rate has been observed. These devices are relatively easy to fabricate from ridge-waveguide lasers or any other lateral confinement laser.

The advantages and the characteristics of the fiber optic gyroscopes (FOGs) for the space application, have been mentioned. There are differences of the FOG designing between application in space and other conditions especially the reliability. The life of space vehicle is usually between 5 years to 10 years . As one important part of the space vehicle, the reliability of the fiber optic gyroscope is critical. In this paper, two super luminescent diodes (SLD) design in fiber optical gyroscope have been discussed. It is necessary to greatly improve the reliability of fiber optical gyroscope for space application. The key techniques of the two SLD design have been proposed. To use two SLD, the design of FOG must be changed , control of the SLD is also important.

The design process of superluminescent diode has been discussed on the basis of nitride diodes with bend‐waveguide geometry. The devices were fabricated by MOVPE technique on bulk GaN substrates and emitted light at the wavelength of around 420 nm. The effectiveness of the cavity suppression design was confirmed by measurements of high‐resolution emission spectra. Further optimization was carried out by appropriate selection of the angles of the waveguide bend and the chip length. Thus; we succeeded in fabrication of high optical power superluminescent diodes with slope efficiency fully comparable to that of laser diodes (greater than 0.9 W/A).Comparison of emission spectra of a superluminescent diode and a laser diode.

A new holography display technology based on a phase-only spatial light modulator (SLM) is proposed. The normal use of laser diode (LD) light source led to the inevitable speckle noise introduced by the coherence of the laser beam. Algorithms and special diffractive optical elements have been proposed to reduce the speckle noise. In this paper, a selected light-emitted diode (LED) light source was used in the holographic display system to replace the LD light source. The temporal coherence and spatial coherence of the LED were studied. Though the temporal coherence of LED is short, the spatial coherence of the light field can be improved by optimizing the optical paths such as decreasing the emitting area of the light source and so on. A high-power LED with a narrow band-width was selected. An algorithm to generate computer-generated hologram for the SLM was proposed. The phase-only holograms for the display were computed using the coherent light algorithm. Then the phase holograms were uploaded to the SLM. LED was used as the light source to illuminate the SLM uniformly, and the reconstructions can be observed by naked eye. It is demonstrated that LED is an acceptable light source for holographic display. The reconstruction results showed that the speckle noise and multiple reflections were eliminated when LED was used as the light source.

Relative intensity noise (RIN) is one of the most significant factors limiting the sensitivity of an optical coherence tomography (OCT) system. The existing and prevalent theory being used for estimating RIN for various light sources in OCT is questionable, and cannot be applied uniformly for different types of sources. The origin of noise in various sources differs significantly, owing to the different physical nature of photon generation. In this study, we characterize and compare RIN of several OCT light sources including superluminescent diodes (SLDs), an erbium-doped fiber amplifier, multiplexed SLDs, and a continuous-wave laser. We also report a method for reduction of RIN by amplifying the SLD light output by using a gain-saturated semiconductor optical amplifier.

Amid the growing global demand for sustainable crop production, plant factories with artificial lighting (PFALs) have gained attention as the systems provide stable, optimal growth environments for crops, and are largely unaffected by climate change. However, a limitation of widely used light-emitting diodes (LEDs) in PFALs is their decreased energy conversion efficiency at high output levels, prompting the search for more efficient light sources. This study focused on laser diodes (LDs), which has shown superior energy conversion efficiency, as an alternative. We developed an LD lighting system capable of mixing red and blue light at arbitrary ratios and mounted it onto a commercial gas-exchange measurement system. Using this system, photosynthetic parameters in rice were obtained under conditions of red light alone and as well as combined red and blue light illumination. Under red LD illumination alone, steady-state CO2 assimilation rate, stomatal conductance, and transpiration rate in rice were significantly higher than those under red LED illumination, whereas intrinsic water-use efficiency decreased due to a relatively greater increase in stomatal conductance. Notably, stomatal conductance and transpiration rate exhibited pronounced temporal oscillations with a period of approximately 900 s, which closely corresponded to oscillations in stomatal aperture confirmed by microscopic observations. Under combined red and blue LD illumination, steady-state photosynthetic parameters did not differ significantly from those under LED illumination; however, the dominant oscillatory frequency observed under red LD alone was not detected, and some photosynthetic capacity parameters tended to decline. Furthermore, CO2 response analyses revealed that, despite lower CO2 assimilation, stomatal conductance responded more strongly to changes in intercellular CO2 concentration under combined red and blue LD illumination. Taken together, these results demonstrate that LD lighting, particularly red LD, enhances stomatal dynamics and induces characteristic oscillatory behavior compared with LED lighting. While red LD appears to be a promising cultivation light source for PFALs capable of maintaining high photosynthetic activity, the physiological impacts associated with blue LD, including potential reductions in photosynthetic capacity, require further study to optimize blue-light proportions for rice cultivation.

Superluminescent diodes represent a device type which can fill the gap between light emitting diodes (LEDs) and laser diodes. The light generation based on Amplified Spontaneous Emission promotes high brilliance of the light source (better than in case of LEDs), and by this efficient light harnessing e.g. coupling to optical fibers. At the same time the low time coherence of the light reduces the interference effects, unfavorable in applications such as Optical Coherence Tomography or in direct image projection. This article reviews the development process and the current status of nitride superluminescent diodes, from their first introduction in 2009 until present.

A broad variety of light sources exhibit photon-count fluctuations that display inverse-square spectral behavior at extremely low frequencies. These sources include light-emitting diodes, superluminescent diodes, laser diodes, incandescent sources, and betaluminescent sources. In a series of experiments carried out over an 18-month period, the photon-count fluctuations for these sources were found to exhibit a $1/f^2$ spectral signature over the frequency range $1.0 \times 10^{-6} \le f \le 5.0 \times 10^{-4}$ Hz, corresponding to $33$ min $ \le T_f \le 11.6$ d, where $T_f \equiv 1/f$. The lower time limit is established by the photodetector noise floor while the upper time limit is determined by the duration of the individual experiments. Scalograms computed from our data are consistent with the periodograms. The universal character of this inverse-square baseband spectral behavior stands in sharp contrast to the optical spectra of these sources, which differ markedly. Time traces of the photon-count fluctuations are examined to shed light on the origin of this enigmatic spectral behavior.

Summary form only given. Broadband operation of a gain-switched Ti:sapphire laser with good beam quality is presented as a function of resonator and pump parameters. This new light source links low temporal coherence and high spatial coherence and can be used as a suitable and simple light source in optical coherence radar measurements. This method is based on white-light interferometry and allows the measurement of the shape and the roughness of optical rough three-dimensional objects without contact or destruction. Conventional light sources like superluminescent diodes or lamps provide too low brightness for large or dark objects in optical coherence radar measurements. Fast tunable lasers or mode-locked lasers are too complex and expensive to be applied in commercial devices for this application. Therefore we set up this simple laser, which emits a broad emission spectrum with high beam quality and high output power.

A novel light source using a laser diode is proposed for optical heterodyne interferometry. The light from this source has a two- frequency beam with mutually orthogonal polarization directions. The light source consists of a laser diode and some optical elements. A Mach-Zehnder interferometer using polarizing beam splitters and a right-angle prism creates orthogonal linearly polarized two-frequency light. The laser diode is easily available to give frequency modulation by current injection. A two-frequency beam is created by giving the time difference due to different optical paths in the Mach-Zehnder interferom- eter. The frequency difference between two beams remains constant regardless of frequency modulation of the laser diode. Moreover, in order to compensate for the power fluctuation by injection current modulation and to keep the total power constant, another incoherent laser diode is added. This light source is suitable for practical applications and requires a smaller and less expensive unit than such conventional light sources as a Zeeman laser or an acousto-optic modulator. This source is applied to a differential interferometer to demonstrate utility for three-dimensional surface profilometry. © 1996 Society of Photo-Optical Instrumentation Engineers. Subject terms: optical heterodyne interferometry; laser diode; frequency modulation; differential interferometer; surface profilometry.