Preheating beams for optical recording

The present invention relates to optical recording using more than one laser beam to record a track.

Optical recording consists of the controlled marking of a media surface using a focused and modulated laser beam. The laser beam heats regions of a recording layer to a media-specific critical temperature. In the regions so heated, the medium undergoes a visible change that may be subsequently detected under lower power illumination.

FIG. 1 illustrates a prior art optical recording process. The output of a recording laser 10 is modulated by data channel electronics 12 . A focusing lens 14 focuses the modulated laser beam to a small recording spot 16 on the surface of an optical recording medium 20 . The medium is scanned relative to the focused spot in a scanning velocity 18 . The recording medium absorbs laser beam radiation and is heated locally in the vicinity of the focused spot. Visible recorded marks 22 form on the media surface at locations that have been heated above the critical temperature.

If the recording medium lacks sufficient sensitivity, or if the recording laser lacks sufficient power, the recording medium will nowhere reach the critical temperature and no marks will form. In FIG. 2 prior art, data channel electronics 12 modulate a laser beam from a low power source recording laser 10 ′. The laser beam is focused by lens 14 to form a recording spot 16 ′ of reduced intensity on the surface of the recording medium 20 . With reduced intensity, the spot heats the media surface to a lesser degree, resulting in recorded marks 24 that are smaller than the recording spot and irregular in size.

For many applications and system designs, the rate of optical recording is limited by the available laser power. It is well known that less recording power is required when the media is already hot. Thus, the effective media sensitivity may be increased by preheating (see U.S. Pat. Nos. 4,383,261 and 4,530,080).

One prior art method of preheating is shown in FIG. 3. A low power beam from recording laser 10 ′ is modulated by data channel electronics 12 . The laser beam is focused by a lens 14 to form a focused spot 16 ′ with reduced intensity at the surface of the optical recording medium 20 . A heating element 28 positioned upstream from the focused spot heats the media surface to a temperature lower than a critical recording temperature. The heated zone 30 of the medium extends at least to the focused recording spot. Because the laser heating is added to the preheating of the medium, the laser pulses create larger recorded marks 26 . However, preheating also causes the mark shapes to be irregular in shape. The shape irregularity arises because local variations in media characteristics or recording system parameters are exaggerated when the thermal gradients across the medium are small during the recording event.

An alternative prior art method of preheating for optical recording is to provide a second focused optical spot that heats the medium immediately upstream from the recording spot, as in FIG. 4. A low power laser beam from recording laser 10 ′ modulated by data channel electronics 12 is focused by the focusing lens 14 onto the recording medium 20 . The beam from a second, unmodulated preheat laser 32 also passes through the focusing lens, creating a preheat spot 34 located immediately upstream from the recording spot. The width of the preheat spot is substantially the same as the recording spot. The preheat spot heats the medium to a temperature less than a critical marking temperature in a heated media zone 30 ′ that extends to the recording spot 16 ′. In order to assure that the preheated media zone extends to the recording spot, the preheat spot must be closer to the recording spot than the scan velocity 18 multiplied by about 2 microseconds. The pulses of the recording laser create recorded marks 26 that have increased size because of the preheating. The irregularity of the mark shapes is reduced because the mark width is confined to the narrow preheated zone. However, preheating still causes some variability in mark length.

Unintended variation in recorded mark length and width degrades signal quality, inducing readout errors and limiting the storage capacity and channel performance of the recording medium.

It is an object of this invention to increase the effective media sensitivity for optical recording without substantially increasing the length irregularity of the recorded marks.

This object is achieved by a method for optical data recording, comprising:

a) focusing a preheat laser beam and a recording laser beam onto an optical recording medium, forming a preheat focused spot and a recording focused spot having nearly equal sizes;

b) providing relative scanning motion between the optical recording medium and the recording focused spot in a direction parallel to the displacement from the preheat focused spot to the recording focused spot at a velocity sufficient to prevent thermal diffusion from the optical recording medium during the transit of the medium from the preheat focused spot to the recording focused spot;

c) modulating the preheat laser beam according to a modulated data signal, thereby creating preheated zones on the media surface with temperatures less than a threshold for complete mark formation;

d) modulating the recording laser beam according to the modulated data signal, with a time delay equal to the separation between the preheat focused spot and the recording focused spot divided by the scanning velocity; and

e) causing the recording laser beam to illuminate the recording focused spot with sufficient power to raise the temperatures of each preheated zone above the threshold for complete mark formation, creating a track of permanent recorded marks.

It is an advantage of the present invention that effective high speed optical recording can be achieved using preheat laser beams without the need for high power laser sources.

It is another advantage that by practicing the present invention, high fidelity recording can be achieved.

It is a further advantage of the present invention that VSCEL arrays may be utilized for multitrack optical recording.

FIGS. 1 and 2 describe some features of the optical recording process according to the prior art;

FIGS. 3 and 4 describe prior art methods of preheating to assist the optical recording process;

FIG. 5 shows how the reliability of preheating may be improved according to the current invention;

FIG. 6 shows another embodiment of the present invention; and

FIG. 7 shows a further embodiment of the invention for multitrack recording.

According to the present invention, media preheating is accomplished by one or more preheat beams that are focused upstream from the trailing record beam and modulated synchronously with the trailing record beam to selectively heat the same regions of the medium. The preheat spots have essentially the same size as the corresponding record spots, scan the same tracks on the media surface, and are modulated with the same data signal. But modulation delays are applied to the spots such that both preheat and record spots heat the same regions of the media. This eliminates the mark variability sensitivity that is inherent in prior art preheating methods.

In one implementation of this invention, both the record laser source and the preheat laser source(s) are elements of a Vertical Cavity Surface Emitting Laser (VCSEL) array 100 . Use of a laser array assures that precise relative alignment can be maintained between the record and preheat spots (both cross-track and in-track). A two-dimensional VCSEL array may be used for multichannel optical recording, with independent synchronized preheating for each track.

Where parts correspond to prior art elements, the same reference numerals may be employed for clarity of illustration.

Referring now to the apparatus shown in FIG. 5, beams from recording laser 10 ′ and preheat laser 32 are respectively focused through the focusing lens 14 onto the recording medium 20 forming a recording spot 16 , and a preheat spot 34 . Both the recording laser beam and the preheat laser beam are modulated by data channel electronics 12 ′. The channel electronics 12 ′ delays modulation of the recording laser beam relative to the modulation of the preheat laser beam, so that each of the preheated media regions 36 created by laser beam pulses from the preheat laser are subsequently illuminated by recording laser beam pulses at the recording spot and thereby heated above the critical temperature, forming recorded marks 26 ′. The extent of critical heating is constrained by the limited width and length of the preheated media regions. Therefore, the length and width variability of the recorded marks is reduced.

It will be appreciated that additional synchronized preheat lasers may be employed if additional sensitivity enhancement is required. In the apparatus of FIG. 6, two preheat lasers 32 a and 32 b produce beams that are aligned to illuminate two preheat spots 34 a and 34 b upstream from the recording spot 16 ′. Both preheat laser beams are modulated by the data channel electronics 12 ′ and synchronized so that they both preheat the regions that will be raised to the critical recording temperature by pulses of the recording laser at the recording spot 16 ′.

FIG. 7 shows a plurality of apparatus, each of which synchronizes preheated beams for simultaneously recording multiple tracks. This arrangement is implemented using a two-dimensional VCSEL array that includes a line of recording laser elements 110 and a neighboring line of preheat laser elements 132 . Beams from the recording lasers are focused by a focusing lens 14 creating a line of recording spots 116 on the surface of an optical recording medium 20 that scans in a direction 18 with respect to the recording spots. Beams from the preheated lasers 132 are also focused by the focusing lens, forming a line of preheat spots 134 , one preheat spot directly upstream from each recording spot. The modulation of each recording laser beam is sychronized with the corresponding preheat laser beam so that both lasers heat the same regions of the recording medium, with the result that recorded marks 130 are formed downstream from each recording spot.

The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

10 recording laser

10 ′ recording laser

12 data channel electronics

12 ′ data channel electronics

14 focusing lens

16 recording spot

16 ′ recording spot

18 scanning velocity

20 optical recording medium

22 recorded marks

24 recorded marks

26 recorded marks

26 ′ recorded marks

28 heater

30 heated media zone

30 ′ heated media zone

32 preheat laser

32 a preheat laser

32 b preheat laser

34 preheat spot

34 a preheat spot

34 b preheat spot

36 preheated media regions

100 VCSEL array

110 recording lasers

116 recording spots

130 recorded marks

132 preheat lasers

134 preheat spots