Search

The use of a holographic optical element (HOE) is proposed as a substitute for the fine-pointing mechanism (FPM) in free-space optical communications. A HOE possessing a function to transform the propagation angle of the received beam to a designed direction is fabricated. The coupling characteristics of the received beam to a single-mode fiber are experimentally evaluated. The results confirm the potential of the proposed method and show the advantages over the FPM.

The imaging characteristics of volume holographic imaging elements (VHIEs) are evaluated numerically and experimentally. To evaluate the imaging characteristics of a VHIE, a volume holographic imaging simulator between one-dimensional regions is fabricated. In the simulator, the propagated light in free space and the diffracted light at the VHIE are calculated by numerical Fresnel propagation and Kogelnik's coupled wave theory, respectively. The imaging characteristics of VHIEs for imaging between point sources and between one-dimensional regions are evaluated. Furthermore, we numerically and experimentally investigate the influences of Bragg-mismatched light on imaging characteristics along the direction of the optical axis of a VHIE.

In this study, a small-form-factor optical pickup (SFFOP), corresponding to the Blu-ray disc (BD) specifications has been developed. The developed SFFOP is composed of an integrated optical pickup and a swing-arm-type actuator. The integrated optical pickup has been developed in the form of an array using wafer-level fabrication technology. The developed SFFOP is composed of the fundamental optics which yields a numerical aperture (NA) of 0.85 and uses a blue laser diode having a short wavelength of 407 nm and a silicon optical bench, which consists of a laser diode, a photodiode and several mounts for the laser diode, an objective lens and mirrors. The micro objective lens is bonded to the lens holder on the SFFOP by an active alignment using a modified Mach–Zehnder interferometer. Also, a static focus error signal was detected to assemble a polarized holographic optical element. Through the measurement of the focus error signal with a swing-arm-type actuator, which was developed for the SFFOP, it is estimated that the developed SFFOP satisfies the BD specifications with the balance of the focus error signal below 10%.

We have developed an integrated optical unit employing a two-wavelength laser diode for a digital versatile disc (DVD)/compact disc (CD) optical pickup. In this paper, we propose a new design method to share a holographic optical element for 780 and 650 nm light beams in an integrated optical unit. To share the holographic optical element, we have introduced an additional aberration to reduce the focusing error signal offset of the 780 nm light beam. Moreover, we have introduced the phase-shift differential push–pull (PS-DPP) method for both the 650 and 780 nm light beams. We have evaluated the performance of this integrated optical unit and confirmed that the performance of this unit is sufficient for both DVD and CD systems.

The laser unit that adopts a polarized hologram element and a front laser diode (LD) monitor of 10.4 mm (width)×4.8 mm (depth)×5.5 mm (height) for digital versatile disc pickup was developed. By optimizing the phase transfer function of the hologram element and the division ratio of six segments of the photodetector, suitable characteristics of sensing signals, which were used in the spot-size-detection (SSD) method for the focusing sensor and the differential-phase-detection (DPD) method for the tracking sensor, were confirmed experimentally. The front monitor method using the reflective hologram was developed for stable control of the laser beam. The developed laser unit achieved an optical efficiency of more than 90%, and a wavefront aberration of 0.014λ rms. This paper describes the optical design of the laser unit system that adopts the polarized hologram element (PHOE) and optical characteristics of the PHOE laser unit.

We developed a new and effective technique for the fabrication of long period fiber gratings (LPFGs) with UV exposure energy by using a holographic optical element (HOE). Our HOE exposure technique produces a stripe line beam whose period and contrast are variable through a computer generated HOE. The HOE exposure technique is twice as efficient in terms of energy as the conventional contact mask method, because the HOE converts a nonuniform input beam into a stripe line beam, while in the conventional contact mask method, the beam loses energy at two steps: the beam homongenizer and the amplitude mask. In order to clarify which beam intensity profile is efficient for LPFG fabrication, the contrast of the stripe line beam profile was varied from 0.12 to 1.33, which is defined as the ratio of the first harmonics to the DC component of the stripe line beam. The result shows that the increase of the peak loss during the LPFG writing process is strongly dependent on the first harmonics amplitude of the stripe line beam profile, not simply on the total exposure energy, though the peak wavelength shift is linear to the total exposure energy. That is to say, the higher contrast beam effectively reduces the exposure energy to achieve the requisite peak loss.

Using a compound prism with a holographic optical element and a birefringence wedge, we have developed a twin-spot method pickup for a Mini Disc player. Since tolerance sharing is optimized by ray trace simulation, servo signal offsets caused by assembly error and fabrication error are compensated simply by positioning and rotating the compound prism in a plane; however, all servo and magneto-optical (MO) signals are detected by a photodiode. Although two half-wave plates are placed in the diverging light beam, it is theoretically and experimentally revealed that they hardly deteriorate the MO signals. The jitter value of the MO signal reproduced by this pickup reached 19 ns.

A novel active filter for transmission light modulation based on a vibration-controlled beam fanning along the c-axis in BaTiO3 is proposed and demonstrated. To the best of our knowledge, it is the most efficient method of controlling the fanning effect in a photorefractive crystal with a large gain in two-wave mixing.

We have suggested methods to achieve read/write compatibility for a high-density (HD) disk of 0.1 mm substrate thickness and digital versatile disk of 0.6 mm substrate thickness by simply adding an optical element. We have confirmed that the polarized holographic optical element (HOE) and polarized phase compensator (PPC) are strong candidates for achieving compatibility.

The use of a multilayer is efficient for increasing the recording capacity of optical discs, but the unnecessary layer cross talk affected by the stray light is also increased. Layer crosstalk should be suppressed to realize a stable tracking servo system. A one-beam push–pull method with a simple holographic optical element for a multilayer Blu-ray disc is proposed, and its effectiveness in suppressing the layer crosstalk was experimentally confirmed.

A polarization-selective multiple-beam liquid crystal grating (MBLCG) was designed. It can be applied to an integrated optical unit (IOU) for a recordable optical pickup head in small form factor with the functions of recording data using a single beam and retrieving data using multiple beams, respectively. This device is constructed using the technologies of liquid crystal light modulator for switching diffraction effects of a single transmissive beam and multiple diffracted beams. The proposed IOU composes of a laser diode, a photodetector, a MBLCG and a holographic optical element (HOE). The optical axes of the MBLCG and HOE are mutually perpendicular so that the round-trip light efficiency of the IOU can be maximized. Experimental results show that the MBLCG is capable of generating seven diffracted beams with equal light intensities and polarization-selective corresponding to an incident beam of the ordinary or extraordinary state.

In this paper, we review a number of digital versatile disk (DVD) pickup designs and present a new structure with two wavelengths using a 780 nm integrated optical unit (IOU) and a 650 nm laser diode (LD) in a single optical path. The 780 nm IOU acts as a sensor for the 650 nm system. Two returned beams of different wavelengths are diffracted by a holographic optical element (HOE) to the same detector. With this construction, the number of components is reduced and alignment is effectively simplified.

A polarization-selective holographic optical element with compact dimensions of 2.4 mm×1 mm×1 mm (width × length × thickness) is proposed. The element includes three modules: a holographic polarizing beamsplitter, a holographic input coupler, and a dove-prism coupler so that a simple and dense optical pickup head can be achieved. Polarization properties of holographic volume gratings are briefly discussed. Based on the results obtained, the fabrication of the element is implemented and described. The performance of the element in magnetooptical pickup heads is demonstrated using a differential detector.

This paper introduces a novel holographic optical element (HOE) pattern design to generate focusing and tracking signal for using in digital versatile disc DVD-multi pickup heads. The digital verstile disk (DVD) multi drive will be able to read disks written in the DVD-ROM, DVD-video, DVD-audio, DVD-R/W, and DVD-R formats. DVD multi drive will also be able to write disks in the DVD-RAM and DVD-RW/R formats. The HOE design provides the focusing error signal and both differential phase detection (DPD) and pushpull tracking signals for DVD-ROM disks and rewritable land-groove disks. The HOE pattern is divided into two main regions. The inner region is for focus detection and the outer region is for DPD and Pushpull tracking detection. In this study, a rotary actuator is used to reduce the tilt angle of the objective lens in the DVD-multi pickup head. The finite element method of the actuator is studied to analyze the resonance of the structure and the magnetic field of the voice coil motor.

A dual-layer volume holographic optical element is proposed as a beam-splitting element for a three-wavelength (λ=0.4 µm, 0.65 µm and 0.8 µm) optical disk pickup. We fabricated it for normal incidence using photopolymer film via an inexpensive laminate process. It is demonstrated that it exhibited excellent wavelength selectivity (extinction ratio ≥∼100) with a high diffraction efficiency of 99.7%. This element could be a key device in a next-generation multi wavelength optical disk pickup.

A digital versatile disc (DVD)-and compact disc (CD)-compatible optical pickup using a two-wavelength-integrated laser diode (TWIN-LD) has been developed. The TWIN-LD has two emission points in one chip, one for the red laser beam to read out signals from DVDs and the other for the IR laser beam to read out signals from CDs that are arranged along the active layer. With the application of a TWIN-LD to the optical pickup, the necessity for four optical components, namely, a laser diode, photodetector, holographic optical element and dichroic prism, is avoided in comparison with conventional optical pickups. This paper discusses some key points for designing an optical system using a TWIN-LD, and the results of the experiments appling the optical system to an optical pickup 7.3 mm in height.

A new focusing error detection method using ±first-order diffracted beams of a holographic optical element (HOE) is proposed. We fabricate the proposed HOE and measured the focus offset between a land a groove. According to the measurement results, the focus offset of the proposed HOE was smaller than that of an astigmatic focusing error detection.

The astigmatic focus detection method and differential phase detection (DPD) tracking method are now widely used in a digital versatile disk (DVD) pickup head. The DPD tracking method requires the returning spot to remain centered in a quad detector and therefore makes the optical alignment very critical. In this paper, a novel holographic optical element (HOE) pattern design to relax the alignment tolerance and achieve a stable DPD tracking servo property is described. Simulations show that the servo signals are insensitive to photo-detector (PD) misalignment when compared to that of a conventional HOE design.

Backward compatibility with CD-R necessitates having two laser sources in the digital versatile disc (DVD) pickup head. The dual-wavelength optical head complicates pickup design and enlarges the volume of the head. Moreover, it increases the number of components and assembly cost. In this paper, we present a new design for a dual-wavelength holographic pickup-head module. Packaged together in the module are two laser diodes, photodiodes and a holographic optical element (HOE) for diffracting reflected light from the disk onto the corresponding photodetectors. This module can be used as the core of an optical head capable of reading DVD, CD and CD-R.

A dual wavelength optical head for 0.6 mm and 1.2 mm substrate thicknesses, containing 635 nm and 785 nm laser diodes, has been developed. Holographic optical element (HOE) type optics and variable magnification type optics are described. For both types of optics, high light power efficiency is obtained, which is suitable for random access memory (RAM) applications. Moreover, the use of 785 nm light makes it possible to read compact disc-recordable (CD-R). A readout operation for a quadruple density CD and a conventional CD has been successfully demonstrated.

The application of vertical-cavity surface-emitting laser (VCSEL) diodes for optical data storage has been investigated. VCSELs suitable for compact disc optical heads have been designed and fabricated. A photo diode for optical power monitoring is integrated in a VCSEL chip, and a holographic optical head using VCSEL is also manufactured. Single transverse mode operation up to 1.5 mW shows good performance.

A compact magneto-optical head module integrated with chip elements, which uses a polarizing grating and a holographic optical element, has been developed. The module size is W14.4 mm×D5.6 mm×H13 mm. A readout experiment for a magneto-optical disk has been carried out employing a 685 nm laser diode. Carrier-to-noise ratio of more than 45 dB has been obtained under quadruple capacity conditions compared with the first-generation ISO (International Organization for Standardization) standard. Alignment tolerance was also investigated.

A polarizing holographic optical element (HOE) with compensating grooves on the surface of a LiNbO3 substrate has good polarizing beam-splitter characteristics. This polarizing HOE was applied to a recordable optical head with an integrated laser and photodetector unit. In the optical head, a correct far-field tracking method is constructed by setting the polarizing HOE and a quarter-wave plate right under the objective lens, which can be moved with the objective lens. A holographic pattern of the polarizing HOE is divided into 20 portions along a disc track direction for spot size focus error detection and far field tracking detection. Wth this optical head, conventional CD play-back and recording signals on phase change discs were performed. Then the tracking offset caused by objective lens shift was measured and determined to be markedly decreased. Signal degradation by the ±1 st light diffracted by a polarizing HOE was not observed. Good characteristics of recording and play-back for phase change discs were also obtained. This recordable optical head was found to have desirable characteristics and high productivity.

In this paper fuzzy analysis for economically fabricating a holographic optical element (HOE) in photoresist is conducted. We find that the grating profile has a close relationship with diffraction efficiency, and that the process parameters influence the grating profile significantly. Thus, using the fuzzy control in these relations, it becomes possible to predict the diffraction efficiency and to solve many problems in the grating fabrication process. From the modulation mechanism in these complex holograms we believe we have extracted the necessary and intrinsic qualities, and our model of fuzzy set analysis produces good general agreement with the experimentally measured diffraction efficiency. The new concepts of the “linear equivalent" which we established to simplify a HOE from a honeycomb structure to a linear one will be helpful for processing various types of HOEs.

A blazed holographic optical element (HOE) was used in the development of an optical pickup for video disc players. In the analysis, an optical utilization efficiency of 13.2% was obtained by optimizing the groove profile, although the HOE includes various grating pitches (from 1.6 to 2.2 µm). The blazed HOE was fabricated on a glass substrate by the ion-milling technique with CHF3 gas etching. The signal-to-noise ratio of the video signal reproduced with this optical pickup was more than 48 dB, and the pickup showed good environmental stability.

A new focus error detecting method for an optical disk drive has been developed using a holographic optical element (HOE). The groove pattern on the HOE was designed using the ray-tracing and least-squares-fitting methods to attain a symmetric beam profile. It has been proven successfully that this method of focus error detection is insensitive to wavelength fluctuations, objective lens shifts, and tracking grooves.

A new holographic optical element, made of a birefringent LiNbO3 substrate, has been developed for magneto-optical, disk heads. The element includes the analyzer function required for magneto-optical signal detection, in addition to focusing and tracking error signal detection functions. The analyzer function was realized by introducing a birefringent grating structure, using proton exchange method. More than 15dB extinction ratios were obtained for the fabricated element.

A high performance optical head, using a holographic optical element, which consists of only four optical elements, has been developed. The relief profile of the holographic optical element has been optimized to obtain high light power efficiency. As a result, the maximum light power efficiency has reached 14.8%. Holograms for detecting tracking error have been optimized to suppress tracking signal off-set. It has been experimentally shown that the tracking signal off-set does not appreciably arise.