GRIN Rod Lens Research Articles

Application of GRIN-lens-based in-line digital holographic microscopy to automatic detection and localization of particles released from a MEMS device.

Recently, we developed a compact and easy-to-implement in-line digital holographic microscope (DHM) using a GRIN rod lens, which provides better resolution (1.3µm) compared with commonly used pinhole-based DHM setups. Here, we employ this microscope to acquire 3D holographically reconstructed images of silica microparticles, within the 10-300µm size range, launched/released from a microelectro-mechanical systems (MEMS) device. To the best of our knowledge, this is the first time that a MEMS device is implemented to store and launch microparticles. The custom-designed MEMS device consists of a 50µm thick flat circular silicon ultrasonic membrane mounted on an off-the-shelf piezoelectric transducer. Moreover, we propose and experimentally demonstrate a new automatic hybrid detection and localization method for particle field holography that benefits from a combination of well-known minimum intensity and variance of gray level distribution focus metrics. This robust method is fast and provides precise d e p t h/z position of particles. The proposed method is applied for particle testing of the MEMS device, reconstructing 3D visualization, and measuring the size and velocity of released particles. The obtained experimental results show that the velocity of released particles, previously dry-loaded onto the MEMS device, is of the order of a few tens of cm/s.

Performance enhancement of an ultrafast all-fiber laser based on an InN saturable absorber using GRIN coupling

Indium nitride (InN)-based semiconductor saturable absorbers have previously shown advantages for application in near-IR fiber lasers due to their broad modulation depth, ultrafast nonlinear response and thermal stability. However, up to now all demonstrated saturable absorber elements based on InN (either transmissive or reflective) have shown limited performance due to poor coupling and insertion losses. We present here a simple mode-locking device based on a GRIN-rod lens in conjunction with an InN semiconductor saturable absorber mirror (SESAM) for its use in a passively mode-locked all-fiber laser system operating at telecom wavelengths. Our results demonstrate that this coupling element ensures not only a compact, turnkey and alignment-free design but also a highly-stable optical femtosecond pulse train. The reduction of insertion losses (3.5 dB) enables the generation of 90-fs ultrafast pulses with an average power of 40 mW and up to 7 nJ of pulse energy without the need for additional amplification.

Microendoscopic Confocal Imaging Probe Based on an LVD Microlens Scanner

This paper reports a microendoscopic confocal imaging probe based on a large-vertical-displacement (LVD) microlens scanner. Photoresist reflow technique is used to form a microlens on a lens holder that is integrated with a LVD microactuator. With a footprint of 3 mmtimes2 mm, the LVD microlens scanner can vertically displace the integrated polymer microlens by 0.7 mm at a dc voltage of 23 V. The LVD microlens chip is directly packaged into a 5-mm-diameter imaging probe. The architecture of the imaging probe is very simple and is just composed of this microlens, a GRIN rod lens (Oslash1.8 mm), a metal mold, and a cover

Analysis of large refractive index difference of GRIN-rod lenses caused by ion-exchanging

The GRIN-rod lenses are made by two steps ion-exchanging, and the refractive index distribution of the sample is measured by the YaMing interferometer. The results show large differences in the refractive index of the samples. The reason for the increase of the refractive index difference by two-step ion-exchanging is discussed. The results of this research may help to produce GRIN materials.

H-ROD: A new and versatile microoptical component

Abstract We present the semi cylindrical H-ROD element as a new and versatile microoptical component. With field assisted ion exchange semi cylindrical index distributions can be integrated in a planar glass substrate. By combination of two identical elements an integrated GRIN ROD lens can be realized. Furthermore a semi cylindrical element can also be utilized for imaging by using internal reflection (H-ROD). Aside of this the H-ROD provides direct access to the filter plane, which offers a wide range of applications.

Lens-coupled laser diode module integrated on silicon platform

Recent interests in the development of multiservice systems have increased the activities be reduced. However, current commercial-grade manufacturing cannot satisfy these needs. The conventional assembly method results in the labor-intensive in optical access networks, such as fiber to the home (FTTH). For these systems to be viable, the optical module assemble costs and package size must process of finding the optimum optical coupling position. In addition, the number and size of parts cannot be reduced due to the difficulty of manipulating them. As a new approach for a lower cost, compact packaging solution, silicon platform technology is very promising. This paper describe a lens-coupled laser diode module integrated on a silicon platform, where a laser-diode, a GRIN rod lens, and single-mode fiber are hybridly integrated on a silicon platform by passive alignment.

Analytic Solutions of the Eikonal Equation for a GRIN-rod Lens 2. Skew Rays

Abstract Analytic solutions of the eikonal equation for a skew ray in a GRIN-rod lens are derived together with those of the ray equation by the perturbation method. It is algebraically shown that the derived solutions agree with the exact ones for a meridional ray in the sech profile and for a helical ray in the helical exact profile. The numerical accuracy is also examined by the Runge-Kutta-Gill method for the sech profile.

Optical Fibre Grating Demultiplexer-multiposition Switch Using a Single Gradient-index-rod Lens

Abstract An optical fibre device using a single gradient-index (GRIN)-rod lens and a rotatable blazed reflection grating is proposed for simultaneously realizing the functions of both demultiplexing and multiposition switching. An example of design using the commercially available GRIN-rod lens is shown for the 0·8 μm band multimode fibre use. An extensive tolerance analysis is also done as a basis for fabricating this device. In addition, the potential of a GRIN-rod lens is clarified in terms of the channel separation and the transmission loss.

Nd:YAG laser delivery system utilizing GRIN rod lenses as focusing-collimating optics

A sequential electromechanical system capable of delivering the radiation coming from a medium power Nd:YAG laser to an array of multimode optical fibres is presented. The system utilizes GRIN-rod microlenses as focusing-collimating optics. A focusing unit positioned at the fibre distal-end, which can work in two different configurations, has also been designed and constructed.

Ray Trace Evaluation Method of a Multimode Optical Fibre Grating Demultiplexer Using a GRIN-rod Lens

Abstract An evaluation method based on a ray trace analysis is reported for a multimode optical fibre demultiplexer using a GRIN-rod lens and a blazed reflection grating. The validity of this evaluation method is assessed by reference to an existing demultiplexer and a fairly good agreement is obtained between the calculated performance and the experimental results for the previously reported device. An extensive tolerance analysis is also reported, to establish a basis for the fabrication of this device. The results of this analysis are given as a map of the 3 dB contour of the transmission loss.

Analytic Solutions of the Eikonal Equation for a GRIN-rod Lens

Abstract A systematic method based on the perturbation method is proposed for analytically solving the eikonal equation for a meridional ray in a GRIN-rod lens. Using this method, the perturbation solutions up to third order are derived together with those of the ray equation. The computational accuracy of the derived perturbation solutions is examined against the exact one for the sech index profile. At the typical numerical aperture 0·45 of a compact disc pickup objective, the accuracy of the second-order perturbation solution is found to be as high as 1/100 waves.

Coupling characteristic analysis of single-mode and multimode optical-fiber connectors using gradient-index-rod lenses

The coupling characteristics of optical-fiber connectors using gradient-index (GRIN)-rod lenses are analyzed for both single-mode and multimode use. We deal with six types of commercially available GRIN-rod lens. In the single-mode case the coupling characteristics are analyzed by the Gaussian beam approximation, and the effects of the deviation in lens length from a one-quarter pitch, the lens separation, and the lateral shift of the lens are clarified. In the multimode case the coupling characteristics are analyzed by the ray-trace method, and the effect of the third-order aberration of the GRIN-rod lens is clarified in addition to those in the single-mode case. In both cases a type SLS 2.0-mm lens is found to be best. By adjusting the lens length to be slightly longer than a one-quarter pitch, the lens separation and the tolerance for the lateral shift of the lens were improved.

Effect of beam power on imaging characteristics of Gaussian beams with a defocusing GRIN rod lens

Imaging characteristics of Gaussian beams through a nonlinear defocusing GRIN rod lens are studied. Analytical solutions of the beam width parameter, the distance of the focal plane and the linear magnification, in terms of the beam power, diffraction and the rod dimensions, are obtained using the variational method and the ABCD law. It has been found that the beam power can change the optical characteristics of defocusing GRIN rod lenses. Some numerical results are given.

Polarisation-independent semiconductor optical amplifier module using twin GRIN rod lenses

A polarisation insensitive semiconductor optical amplifier module with a gain of 18 dB and a 0.5dB gain difference is demonstrated. Coupling optics using twin GRIN rod lenses achieve efficient and reflection-free couplings.

Pattern edge color difference in a color image sensor with a GRIN rod lens

The color distortion arising around pattern edges of pictures scanned by a color image sensor using a GRIN rod lens is simulated by a model that uses a Gaussian distribution function as the spread function of the lens for two color separation systems. In the time division system, the maximum permissible DeltaH(B) which indicates color aberration is obtained. Although this value depends on some sensor parameters, such as pixel size and pixel period, the method to obtain the maximum permissible DeltaH(B) presented in this paper is suitable in almost all cases. In the space division system, the effect of color aberration on color distortion is clarified, and it is seen that permissible color aberration is larger than that in the time division systems.

Coupling efficiency evaluation of multimode fiber devices using GRIN rod lenses

For an efficient analysis of the coupling characteristics of a multimode fiber device using GRIN rod lenses, it is important to consider both the third-order aberration effects of a GRIN rod lens and the actual source fiber optical power distribution. In this paper the ray tracing technique has been used with a set of point sources representing the extended source fiber core. Such a new representation is carried out according to the best agreement with measurable near-field and far-field source fiber distributions. In such a way we determined a set of results that show the relationships between coupling efficiency, lens length, and fiber lateral misalignments, as a function of the radial position of the input fiber. The results obtained pointed out, on the one hand, the poor reliability of the predictions based on a perfectly parabolic profile and, on the other, the dependence of the coupling efficiency vs lateral misalignment relationships on two different excitation distributions: uniform and steady state. The results of the experimental measurements carried out on a 0.5-pitch GRIN rod lens are consistent with the suggested model. Such an approach can be advantageously used in the design of microoptic devices based on GRIN rod lenses, such as multi/demultiplexers, directional couplers, and lens connectors.

Injection-locked semiconductor laser array using a graded-index rod: a computational model

We describe a computation which models the coupling among five widely separated laser emitters in a single monolithic array due to a GRIN rod lens. Individual emitter modes are characterized as to nearfield rectangular beam waist sizes by using an effective index waveguide calculation. The differential equations describing ray propagation in a medium with nonuniform index of refraction are solved using a Runge-Kutta algorithm. A ray bundle originating at each emitter is traced through free space, the GRIN rod, and back to the array. The fraction of (1/e**2) power entering an emitter from the front then defines the coupling from the source diode to the target diode. Provisions for describing array distances, offsets, and tilts are included. The model was tested for an array with 6-microm stripes on 250-microm centers. An array position and orientation was found which coupled all five laser elements together. The computed coupling coefficients were commensurate with values reqired to fully couple the array elements. Previous experimental results are now interpreted as a result of injection locking facilitated by the rod lens as opposed to an external cavity configuration.

Efficient highly stable laser diode module for single-mode fiber employing a combination of hemispherical ended GRIN rod lens and virtual fiber

An efficient and practical laser diode module is demonstrated for single-mode fiber applications. The module is developed through use of an earlier proposed combination lens method, where a hemispherical ended graded index rod lens and virtual fiber are employed. Since lens and virtual fiber misalignment tolerances are large in this coupling method, the YAG laser welding technique is easily applicable. This laser diode module has not only a simple structure but also high coupling efficiency, excellent reproducibility, and extremely high reliability.

Double exposure speckle technique for imaging system analysis and use in rod lens arrays

New double exposure speckle techniques for imaging system analysis are developed and their feasibility is experimentally examined. Using a single convex lens as the test lens, the MTF of the lens is obtained from the measured visibility of Young's fringes which have been produced by Fourier transforming a double exposure specklegram recorded through the test lens. By using conventional optical filtering techniques the spatial variations in the imaging quality are visualized for GRIN rod lens arrays.

Aberration of GRIN—rod lenses in single—mode optical fiber devices

AbstractGraded refractive index rod (GRIN‐rod) lenses are used widely in optical devices for optical fiber communication systems. Insertion loss is a major criterion for evaluating optical devices, especially those used in single‐mode fiber systems, and aberration is a dominant factor in insertion loss. Thus, aberration analysis is essential in designing high‐perfomance optical devices. This paper presents ray traces within the third‐order aberration when light propagates through GRIN‐rod lenses. Using the fact that the ray is the orthogonal trajectory of the wavefront, we derive a novel wave optical description of GRIN‐rod lenses to the degree of third‐order aberration. This paper shows the relatinship between coupling efficiency and lens parameters. The necessity of considering aberration effects in designing low insertion loss optical devices is shown. The experimental results are in very good agreement with the calculated values, and the validity of the present analysis is confirmed. This paper proves that GRIN‐rod lenses are superior to ball lenses in terms of aberration, and provide higher coupling efficiency.