Edward C. Gage

Control of optical-feedback-induced laser intensity noise in optical data recording

The usefulness of semiconductor lasers can be greatly limited when the laser is subjected to uncontrolled optical feedback (OFB). In particular, the laser intensity noise can be severely degraded when OFB is greater than 0.1%. Although the technique of high-frequency injection (HFI) can solve this problem, the proper modulation frequency and depth must be chosen empirically. We investigate this problem through cornputer simulations of the multimode stochastic rate equations, modified to include OFB and HFI. By providing the program with measurable laser and system parameters, the simulations predict the HFI modulation frequency and depth that optimize the laser behavior. The results of the simulations are compared with experiment, and good agreement is obtained.

Optical-feedback-induced chaos and its control in semiconductor lasers

In this paper we describe some of the effects of external optical feedback (OFB) on semiconductor lasers by simulation of the stochastic rate equations. Particular attention is paid to the lasers transition to optical chaos. In addition, we describe three techniques for avoiding this chaotic regime. The technique of high frequency injection, used in optical recording, can delay the onset of chaos till very high values of OFB. Experimental results are given and are in excellent agreement with the theory. A second technique called occasional proportional feedback can be used with some success to stabilize the chaotic output of semiconductor lasers. The final technique for controlling chaos consists of the optimization of various system and laser parameters so that the laser is least susceptible to OFB.

Directional asymmetries due to write‐laser mode hopping during optical recording

The wavelength of semiconductor lasers generally increases with increased output power due to changes in laser gain with current and temperature. Time‐resolved studies of the optical spectrum during the write‐pulse train clearly depicted the quasirandom mode hopping that occurred. In optically dispersive systems, the wavelength shifts caused significant write‐spot displacements down at the recording surface and affected mark lengths. Thermal modeling studies successfully duplicated the salient features of this effect and provided explanations for some of the experimental observations.

Write noise from optical heads with nonachromatic beam expansion prisms

The purposes for achromatizing the optics in an optical recording head are: (a) to reduce focal shifts of the spot at the disk and in the focus sensing system, and (b) to eliminate lateral shifts of the spot at the disk when the laser wavelength shifts. Reference 1 discusses the use of a very low dispersion glass in the fabrication of molded singlet lenses for use in optical data storage. This paper discusses an effect of a lateral shift of the focal spot that occurs, due to the dispersion in the beam expansion prism, when the laser mode hops.

Heads and Lasers

Publisher Summary The Magneto-Optical (MO) optical head is the transducer in a MO optical disk drive. It must provide a small laser spot that is of <1.0 μm full width half maximum diameter at the recording media surface and maintain that spot at focus to <±0.5 μm and on the data track to <±0.1 μm. It must sense the very small Kerr rotations of the linear polarized light reflected from the magnetized domains, within the data track with a high signal-to-noise ratio. It must also record domains with alternating magnetization into the data track. The challenge is great, but it is met by a combination of a high-power single spatial mode laser diode, a high-performance voice-coil actuator, high quality optics, and detectors packaged together and supported with preamplifiers, laser driver, servo, and channel electronics.

Dual-laser writing scheme for optical data storage

A dual-laser writing scheme is proposed in which an unmodulated short-wavelength read laser augments the writing process effected by a longer-wavelength laser. Apart from increasing the thermal efficiency of the laser marking process itself, the dual-beam writing scheme is shown to decrease the mark width and the recorded mark-length variability. Coupled with the increased resolution of the short-wavelength read spot, these enhancements could improve the performance and packing densities of media.

Low-noise high-reliability 680-nm optical head enables robust 14.8-gigabyte/disk product

A high reliability and low noise 680 nm optical head is described. This head enables a high data integrity, 14.8 GByte/disk product (KODAK optical disk system 2000). The laser intensity noise is measured to be -133 to -137 dB/Hz at the data frequencies. This allows phase margin for the system to exceed 50%. The laser with drive electronics is shown to have a pulse lifetime of greater than 40,000 hours at exaggerated drive conditions.