Two Dimensional Laser Diode Array

In order to record high data rates in optical data storage, multi-channel recording can be used. The existing multi-channel (also known as multi-spot) systems use a laser diode array in which many laser diodes are assembled on a common substrate and their output collected by a single collimating lens. Due to the limited field coverage of high numerical apperature lenses, these arrays can only cover a small width which limits the number of laser diodes which can be used or requires spacings too small to be practical. The current invention overcomes this limitation by combining a large number of discrete laser diodes, each one with its own collimating lens.

In accordance with the present invention, an optical system is provided which utilizes a two dimensional array of discrete laser diodes to record information on the surface of a light sensitive recording medium. Each laser diode has its own collimating lens thus the overall dimension of the array is not limited to the field of view of any lens. This array is imaged onto the recording medium through the scanning lens. Since the distance between the individual laser diodes is large, the apparent distance between the array and scan lens has to be large. For mechanical packaging reasons this distance can be significantly reduced by using a reverse telescope between the array and the scan lens. The distance will be divided by a number equal to the power of the telescope.

To further reduce the distance from the array to the scan lens, the laser diodes have to appear closer than their mechanical separation. To achieve that the diodes are arranged in a staggered two dimensional array where each row is staggered by d/n in the row direction, where d=the distance between columns, n=number of rows. This enables the array to be placed n times closer to the scan lens.

In the drawings:

FIG. 1 shows the method of imaging the array onto the recording medium

FIG. 2 illustrates the two dimensional staggered arrangement of laser diodes in the array

FIG. 3 is a cross section of the array showing the laser diodes and their collimating lenses.

In FIG. 1 an array 1 consisting of plurality of laser diodes 2 arranged in a two dimensional array of n rows by m columns (illustratively 3.times.3). In order to image this array down at the correct size onto recording medium 6, three positive lenses 3,4 and 5 are used with focal lengths of f.sub.1, f.sub.2 and f.sub.3 respectively. The array itself is shown in FIG. 2. The m columns have a horizontal spacing of d and the n rows have approximately the same spacing. In order to write continuously the recording medium 6 has to move relative to scan lens 5. This direction of movement will be referred to as the scan direction. The apparatus spacing between the laser diodes 2 in FIG. 2 is d/n in the scan direction. By way of example, referring to FIG. 2, the first row of diodes appears at a distance d from each other when viewed from the scan direction. The second row is offset by an amount of d/3 when viewed from the scan direction and the third row is offset 2d/3 when viewed from the scan direction. Thus the diodes appear at a spacing of d/3 when viewed from the scan direction.

Referring back to FIG. 1, all laser diodes 2 are pointing approximately to the centre of lens 3. Since the diodes appear at a separation of d/n when viewed from the scan direction, their angular separation is d/nl. This separation is further reduced by lenses 3,4 forming a reverse telescope with a magnification of f.sub.1 /f.sub.2 when spaced f.sub.1 +f.sub.2 appart. Thus the angular separation between laser diodes appears f.sub.1 /f.sub.2 times d/nl=f.sub.1 d/nf.sub.2 l when viewed through lenses 3,4. The distance between lens 4 and 5 is not critical since the light beams can be adjusted to be collimated when going from lens 4 to lens 5. The angular separation f.sub.1 d/nf.sub.2 l is converted to a spatial separation of f.sub.1 f.sub.3 d/nf.sub.2 l by the scan lens 5, having a focal length f.sub.3.

By way of example, for a 3.times.3 array shown in FIGS. 1 and 2 having d=10 mm, 1=300 mm, f.sub.1 =5 mm, f.sub.2 =200 mm, f.sub.3 =4 mm the spacing between the recorded tracks will be f.sub.1 f.sub.3 d/nf.sub.2 l=5.4.10/3.200.300 mm=1.1 microns.

Referring now to FIG. 3, the method of collecting the light from each laser diode is shown. Laser diode 2 is mounted to the array 1 using a clamp 7 and two screws 8. The array can be moved sideways by a small amount to compensate for manufacturing tolerances. The light from each laser diode 2 is collected by a collimating lens 10 mounted in a sleeve 9. Sleeve 9 can be moved in and out of array 1 to achieve the desired divergence of the output beam. Parts 1,9,7 and 8 are made of a stable metal, by way of example, brass.

In operation, each one of the sleeves 9 is adjusted until the beam between lens 4 and lens 5 is substantially collimated. The exact lateral position of each laser diode 2 is adjusted until all light beams substantially pass through the entrance pupil of the scan lens 5.

It is clear to anybody versed in the art that in order to utilize the current invention, means of generating scanning motion between the scan lens and medium are required. These means are of conventional nature, as currently used in optical discs and optical tape recorders.