The present invention relates in general to diode lasers. More specifically, the invention relates to a method of modulating output light from a diode laser, to a method of writing information to an optical disc comprising outputting a light signal from a laser diode device, and to an apparatus for producing a modulated light signal, said apparatus comprising a diode laser device.
Diode lasers are an important part of optical storage applications. It is to be expected that dual-stripe diode lasers producing light having a wavelength in a region about 405 nm (blue light) will be available in the near future. Dual-stripe lasers consist of a high-power diode laser and a low-power diode laser in the same mount, separated by a few hundred microns.
Due to their open cavity (that is, a cavity with low-reflectivity facets), high-power diode lasers lend themselves well to the injection of light from, for example, the low-power laser. In this way the high-power laser may be forced, for example, to emit at the wavelength of the low-power laser (wavelength locking).
Write-once read-many media (so-called recordable discs, such as, for example, CD-R and DVD+R) incorporate a dye layer onto which the information is written in the form of a sequence of marks (also referred to as pits). Traditionally this is done by means of a high-power diode laser (DL) of which the current (and hence the output power) is modulated in accordance with a well-defined scheme, the so-called write strategy. Here the amount of energy absorbed by the dye is primarily determined by the output power of the DL. A mark is written by the deposition of enough energy for changing the chemical composition of the dye.
It is therefore desirable, amongst other things, to reduce the amount of output power required to write information to an optical disc.
According to one aspect of the present invention, there is provided a method of modulating output light from a diode laser in which a first diode laser device injects light into a second diode laser device, the second diode laser device producing an output light signal, characterized in that the output light signal has a modulated wavelength dependent upon the injected light from the first diode laser device.
According to another aspect of the present invention, there is provided an apparatus for producing a modulated light signal, the apparatus comprising a first diode laser device operable to produce a first output light signal, and a second diode laser device operable to produce a second output light signal and arranged to receive the first output light signal from the first diode laser device as an input light signal, characterized in that the second output light signal has a wavelength modulated in dependence upon the first output light signal.
In accordance with another aspect of the present invention, a new write mechanism is proposed in which it is not the output power of the high-power diode laser (DL) that is modulated but its wavelength. The wavelength dependence of the dye absorption translates the DL wavelength modulation into variations of the absorption.
It will be appreciated that the principles of the present invention can be applied to modulating the output from a diode laser device of any suitable wavelength, dependent upon the application of the device. Furthermore, although the device mentioned above is a dual-stripe device, the principles of the present invention can be applied to any laser diode device.
In addition, although the optical data storage scheme described above uses a dye-based disc, it will be readily appreciated that the principles of the present invention can be applied to any storage technique in which the absorption of the recording medium changes with the wavelength of the incident light signal.
These and further aspects and advantages of the invention will be discussed hereinafter with reference to the appended Figures, where
The emission spectrum of the SL close to threshold is shown in the upper part of
Generally, the wavelength of low-power diode lasers is shorter than that of their high-power counterparts. This is symbolized in
As the wavelength of the SL 2 is switched between the free-running value λSL and the injection-locked value λML, the absorption of the emitted light from the SL 2 also changes. This is due to the fact that the wavelengths are situated on the slope of the dye absorption curve.
Without injection from the ML 1, a certain amount of light from the SL 2 is absorbed in the dye layer. This amount is proportional to the absorption coefficient α(λ=λSL) at the wavelength of the SL 2 (without injection). The dependence of a on the wavelength is shown in
The use of a method embodying the present invention moves the wavelength of the high-power slave diode laser (DL) in the direction of shorter wavelengths, see
In this way the constraints on what type of high-power DLs can be used are also relaxed.
Furthermore, adapting this implementation means that information can be written to recordable discs without modulating the current of the high-power diode laser. It is the current of the low-power diode laser (and hence its emission power) that is modulated, and via injection this is translated into wavelength variations of the high-power diode laser.
It is noted that the present invention may be applied not only to diode lasers producing light having a wavelength in a region about 405 nm (blue light), but also to diode lasers producing light at alternative wavelengths such as, for example, about 650 nm (red light) and about 780 nm (infrared light).
Number | Date | Country | Kind |
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02080425.8 | Dec 2002 | EP | regional |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/IB03/05342 | 11/20/2003 | WO | 6/15/2005 |