Yoshitada Katagiri

Supersmall flying optical head for phase change recording media

A supersmall optically switched laser (OSL) head is proposed. A laser diode attached to an air bearing slider forms a complex cavity together with a recording medium, and its light output is detected by a photodetector placed at the other end of the laser facet. Data signals and track error signals read from the sampled servo marks are successfully detected. The signal amplitude variation caused by the flying height change is much reduced, and the SNR is increased to 36 dB (40 kHz to 20 MHz for a phase change medium) by decreasing the reflectivity of the laser facet facing the medium to <5%.

Ion beam sputtered (SiO 2 )x(Si 3 N 4 ) 1− x antireflection coatings on laser facets produced using O 2 –N 2 discharges

The chemical composition of a (SiO(2))(x)(Si(3)N(4))(1-x) film produced by ion beam sputtering was precisely controlled by the ratio of O(2) and N(2) flow rates under a discharge current kept constant to within an accuracy of +/-0.05 A. The reproducibility of the refractive index was improved to +/-0.01. This film was applied to form antireflection coatings with extremely low reflectivity on facets of 830-nm AlGaAs double heterostructure lasers. The minimum reflectivity was 6.8 x 10(-5), and a reflectivity of 1 x 10(-4) was achieved reproducibly. Experimental studies show that antireflection coatings are effective for suppressing the interferometric light output variation of composite cavity lasers.

Applications of an extremely short strong-feedback configuration of an external-cavity laser diode system fabricated with GaAs-based integration technology

A composite-cavity laser diode is used to monitor the reflectivity or the displacement of the external-cavity mirror for micromechanical photonics devices. Optical disk bits are read out in the near field from the difference in medium reflectivity with an antireflection-coated laser diode and a photodiode. Microbeam vibration is also detected in the near field from the phase difference with an uncoated laser diode and a photodiode. In both cases the carrier-to-noise ratio is very high (more than 45 dB) because of the lack of mode-hopping noise resulting from the extremely short (less than 3 µm) external-cavity length and strong light feedback. These composite-cavity laser diode microdevices are fabricated on a gallium arsenide substrate to eliminate the need for optical alignment.

Read/write performance and reliability of a flying optical head using a monolithically integrated LD–PD

A flying head using the switching characteristics of an optically switched laser diode is shown to have an excellent read/write performance and head-disk interface. This head consists of an aluminum nitride slider and a 1.3-microm wavelength InGaAsP laser diode monolithically integrated with a photodetector (LD-PD). A ridged waveguide is fabricated on top of the LD facet to converge the light beam. The lensless flying head has a minimum power of 18 mW for writing information (at 1 MHz) at a medium velocity of 7 m/s. The disk has a SiN protective layer over a SbTeGe phase-change recording medium. The head signal remains good after more than 5 x 10(4) contact start/stop cycles and 4.4 x 10(8) passes in an experimental room environment.

Increased spatial frequency in interferential undulations of coupled cavity lasers

Increased spatial frequency is experimentally observed in the interferential light-output undulation of coupled cavity lasers that use a Fabry-Perot laser diode. The frequencies correspond to undulation periods of λ/4, λ/6, λ/8, and λ/10, which are extremely short compared with the normal period of λ/2. This increase is explained theoretically with a multiple-mode model in which one of the longitudinal modes of a coupled cavity laser with the lowest lasing threshold is selected as a lasing mode. This theoretical explanation is confirmed through experiments with a distributed feedback laser that shows a strong single-mode oscillation and yields light-output undulations with a spatial period of λ/2.

Flying head read/write characteristics using a monolithically integrated laser diode/photodiode at a wavelength of 1.3 μm

A feasibility study is shown for an optical disk drive using an optically switched laser (OSL) flying head. This head consists of an AlN slider and 1.3-micrometers wavelength InGaAsP laser diode monolithically integrated with a photodiode. The laser diode has a taper-ridged waveguide on the top of the facet facing the recording medium. The disk consists of an SiN dielectric protective layer, an SbTe Ge recording medium, and 1.5-micrometers -track pitch sampled servo format pits on the glass substrate. The lensless near-field flying head gives excellent read/write performance and head/medium reliability: a high-quality readout SNR of 35 dB, stable write operation with 16 mW of laser power, and a lifetime of 4.4 X 108 passes under experimental room conditions.

Dynamic microforce measurement by distortion detection with a coupled-cavity laser displacement sensor stabilized in a mechanical negative-feedback loop

A small displacement sensor for dynamically measuring small forces is presented. Employing a coupled-cavity laser configuration constructed with a moving external mirror, this sensor provides high sensitivity with a minimum detectable displacement of 0.8 nm, independently of the lasing wavelength, while the lasing state is mechanically stabilized by a negative-feedback loop to maintain a constant external-cavity length. The sensor is shown to be of great use for detecting extremely small distortions in a stiff cantilever, reflecting transient variations in small friction forces in mechanical systems.

Improvement in signal-to-noise ratio of a longitudinally coupled cavity laser by internal facet reflectivity reduction

Improvement in signal-to-noise ratio (SNR) is described for 830 nm AlGaAs double heterostructure lasers longitudinally coupled to a short complex cavity formed by a laser facet and an external mirror separated from the facet by a few micrometers. The SNR degraded by external cavity length (spacing) variation is improved by reducing internal laser facet reflectivity using an anti-reflection coating. A variation ratio, newly defined to evaluate light output variation due to spacing, reaches 1.6 at a reflectivity of 6.8×10-5. The SNR degraded by laser intensity noise is improved to 56 dB by biasing the drive current at an optimum corresponding to the noise valley which appears for lower laser facet reflectivity. The SNR degraded by temperature variation is improved by linear current control, maintaining a maximum value of it from 10 to 45°C.

Optical heads based on coupled-cavity laser diode

Optical heads wherein a laser diode (LD) is optically coupled through a small aperture to a recording medium acting as an external mirror are described. A flying head and a fiber- coupled head are presented, and their high-sensitivity, wavelength-free detection characteristics are demonstrated. A micro-lens head is also investigated. Interference undulations of the coupled-cavity laser with various spatial frequencies due to variations in the external-cavity length are completely explained. Anti-reflection (AR) coatings on the LD facet with reflectivities of better than -40 dB greatly suppress these undulations. To achieve practical heads, fundamental fabrication techniques including AR coating and monolithic integration on laser wafers are briefly mentioned.