Optical print head

An optical print head for recording on a medium includes a plurality of lasers having laser emissions within a desired wavelength range, an optical fiber adapted to receive combined light from the plurality of lasers at a first end and to emit combined output light at a second end, and includes a hybrid optical element optically coupled to the second end of the optical fiber and adapted to focus the combined output light within the desired wavelength range on the medium.

CROSS-REFERENCE TO RELATED APPLICATION

This application is related to co-pending and commonly assigned application Ser. No. 11/520,514, filed on the same date herewith (Sep. 12, 2006), the entire disclosure of which is incorporated herein by reference.

TECHNICAL FIELD

This invention relates generally to optical recording and more particularly to optical print heads.

BACKGROUND

Optical recording technology that enables consumers and others to record laser-written labels on specially coated recordable CD and DVD media has enjoyed notable commercial success. In light-activated thermal label-recording technology, a surface of the medium is coated with a writable layer of a material that changes appearance when it absorbs laser light of a predetermined wavelength. The color change interaction in a thermochromic imageable coating is enabled by phase transitions of the coating materials occurring at elevated temperatures. These phase transitions do not occur (and, so color does not develop) until the coating temperature reaches a certain value specific to the coating material. If the coating is irradiated with laser energy density that is not high enough to reach the phase transition, the color is not developed. Thus, if a writable layer is exposed to laser radiation with an irradiance distribution in which significant portions have insufficient irradiance to reach the color-forming (phase transition) temperature, some of the energy of the laser radiation is wasted. When relatively high-power laser radiation is required, cost increases can occur due to disproportionately higher laser cost. When multiple laser wavelengths are required, such as for color recording, differences in focal distance for the various laser wavelengths may require optics compatible with a focusing servo system. Thus, there is a need for further improvement in marking of media.

DETAILED DESCRIPTION OF EMBODIMENTS

For clarity of the description, the drawings are not drawn to a uniform scale. In particular, vertical and horizontal scales may differ from each other and may vary from one drawing to another. In this regard, directional terminology, such as “top,” “bottom,” “front,” “back,” “leading,” “trailing,” etc., is used with reference to the orientation of the drawing figure(s) being described. Because components of the invention can be positioned in a number of different orientations, the directional terminology is used for purposes of illustration and is in no way limiting. Similarly, for purposes of illustration but in no way limiting, optical diagrams may be drawn to non-uniform scales and may show elements with non-proportional dimensions.

The terms “recordable medium” and “recordable media” as used in this specification and the appended claims refer to media capable of having information recorded thereon by exposure to optical radiation such as laser light. Such recordable media may include, for example, a compact disk (CD), a digital versatile disk (DVD), an HD-DVD, a Blu-ray Disc™ (BD), a holographic versatile disk (HVD), or a video disk, but are not limited to such forms. Recordable media may also include such media having pre-recorded information readable from at least one side and having an optically-recordable coating on at least the other side for writing a label on the media. The term “recording” means recording or printing a label or other information on a recordable medium such as an optical storage disk.

One aspect of the invention provides embodiments of an optical print head including a number of lasers having laser emissions within a desired wavelength range, an optical fiber receiving combined light from the lasers at one end and emitting combined output light at its other end, and including a hybrid optical element optically coupled to the exit end of the optical fiber. The hybrid optical element is adapted to focus the combined output light within the desired wavelength range on a medium for recording. For many embodiments, the optical fiber may be a single-mode optical fiber. For example, when a single-mode fiber is used, not all the modes of a multi-mode laser are propagated by the optical fiber.

FIG. 1schematically shows an optical diagram of a first embodiment of an optical print head100. A number of lasers, such as the three lasers110,120, and130shown, have laser emissions within a desired wavelength range. The desired wavelength range may include wavelengths between about 365 nanometers and about 1600 nanometers, for example. The laser emissions of lasers110,120, and130may be directed substantially parallel to each other in parallel beams, the parallel beams being optically combined into a beam of combined light. WhileFIG. 1shows three lasers, any convenient number of multiple lasers may be used. Depending on the application, the laser emissions of the lasers may have various different wavelengths within the desired wavelength range, or they may all have substantially the same wavelength, e.g., 780 nanometers. Such a monochromatic laser light source can provide a higher power combined output light without the disproportionately higher cost of a single high-power laser.

An optical fiber150receives combined light from the lasers at one end155and emits combined output light at its other end160. The laser emissions from lasers110,120, and130are combined and optically coupled to optical fiber150by coupling lenses115,125, and135respectively, using one or more mirrors140or their equivalents if needed to direct the light toward the entrance end155of optical fiber150. Mirrors140may be one or more dichroic mirrors to combine light from the various lasers into a combined beam. Each coupling lens115,125, and135optically coupled with its respective laser may be movable in a direction substantially parallel to its own optical axis for focusing. Automatic-focusing-servo arrangements such as those using “voice-coil” actuators for moving lenses are known in the art.

Optical fiber150may be, for example, a photonic crystal fiber (PCF). The photonic crystal fiber is adapted for single-mode operation in a wavelength range including the desired wavelength range, e.g., a wavelength range including wavelengths between about 365 nanometers and about 1600 nanometers. Such a single-mode optical fiber150has a mode field area substantially independent of wavelength. The mode field diameter of the optical fiber150may be made equal to or larger than a desired recording track width, e.g., about 20 micrometers.

A hybrid optical element170optically coupled to the exit end160of the optical fiber150focuses the combined output light within the desired wavelength range into a spot195on a recording medium190for recording. Hybrid optical element170has a diffractive portion175and a refractive portion180represented schematically inFIG. 1by digital features and a curved surface respectively. Thus, hybrid optical element170may include a single lens having a first surface175formed as a diffractive surface and having a second surface180formed as a refractive surface.

While the combined output light from optical fiber150is shown inFIG. 1as being affected first by diffractive portion175and secondly by refractive portion180, hybrid optical element170is not limited to that specific arrangement, or even to separating the two functions (diffractive and refractive) into separate surfaces. In some embodiments, diffractive portion175and refractive portion180may be combined at a single surface of hybrid optical element170. In other embodiments, the combined output light from optical fiber150may be affected first by refractive portion180and secondly by diffractive portion175.

AlthoughFIG. 1shows the laser, optical fiber, and lens as being aligned to combine coaxially, in practice the individual lasers and/or lenses may be oriented to project their light at small angles to the optical fiber axis in order to prevent an unwanted amount of reflected light from returning to the laser after reflection from the medium, which could otherwise cause undesired side effects, such as oscillation in the source laser.

Various embodiments may include one or more sensors such as photodiodes to detect light reflected from the medium. When the optical print head is used to record digital data on an optical storage disk, for example, the sensor may be used to read the data recorded and/or to follow a track on the recording medium.

In some embodiments, such as the embodiment ofFIG. 3, the combination of a beam splitter and quarter-wave plate may be used to guide the reflected beam to a sensor and prevent the reflected beam from returning to the source laser. For example, laser light propagating from left to right inFIG. 3and incident on the quarter-wave plate335after passing through the beam splitter330is linearly polarized, and after passing through the quarter-wave plate it is circularly polarized. Reflection from medium190reverses the sense of the circularly polarized light. That circularly polarized light propagating from right to left inFIG. 3is converted to linearly polarized light in its second passage through the quarter-wave plate, but with a polarization at right angles to the polarization it had previously when propagating in the original left-to-right direction. Thus, this linearly polarized light is reflected in the beam splitter and directed downward along the light path toward sensor350. Thus, the quarter-wave plate is configured to direct the light reflected from the medium to the sensor350.

At least some of the embodiments described herein are believed to operate in accordance with this partial description ofFIG. 3. However, the invention should not be construed as being limited to the consequences of any particular theory of operation.FIG. 3is described in more detail below.

Hybrid optical element170is not necessarily a simple monolithic lens element.FIG. 2shows a portion of a second embodiment of an optical print head, in which the function of hybrid optical element170is performed by a combination of hybrid optical elements210and240cooperating to provide a desired demagnification of the laser light from exit end160of the optical fiber150, with desired effective numerical apertures (NA) to efficiently collect combined laser light from optical fiber150on one side and to form a focused spot195of suitable diameter on recording medium190on the other side, with suitable working distances on each side. The first discrete lens210of this optical arrangement may have a diffractive portion220and a refractive portion230as shown, represented schematically by digital features and a curved surface respectively as inFIG. 1. Similarly, the second discrete lens240of this optical arrangement may have a diffractive portion250and a refractive portion260as shown. Thus, hybrid optical element170may include a number of hybrid lenses, each lens having a first surface formed as a diffractive surface and having a second surface formed as a refractive surface. Hybrid optical element170may advantageously be made substantially achromatic for wavelengths within the desired wavelength range. Hybrid optical element170may also be made free of spherical aberration.

For a focused spot195with diameter matching a recording track width of about 23 micrometers, for example, the optical arrangement ofFIG. 2may have an entrance numerical aperture (NA) of about 0.05 to match the exit NA of optical fiber150and may also have an exit NA of about 0.05, for example.

FIG. 3(partially described above) schematically shows an optical diagram of a third embodiment of an optical print head. This embodiment has two lasers305and310having laser emissions within a desired wavelength range, at least one beam splitter330, and at least one sensor350. Lasers305and310may be diode lasers as inFIG. 1. As described above, quarter-wave plate335may also be included, positioned between beam splitter330and the recording medium190as shown. The beam splitter330is disposed to direct a portion of light reflected from the medium for recording to the at least one sensor. A lens345may be provided to focus reflected light on sensor350. The initially separate laser beams315and320from lasers305and310respectively pass through beam splitter330and quarter-wave plate335(if present) and are combined by hybrid optical element210into a single focused spot195on recording medium190. Hybrid optical element210may be equipped with actuators215, providing motion parallel to its own optical axis for focusing with an automatic-focusing-servo system. Actuators215may be voice coils, for example, or their functional equivalent.

The optical fiber150ofFIGS. 1 and 2may be included in the embodiment ofFIG. 3between beam splitter330and lens210to carry the combined laser beams315and320to lens210for focusing into single spot195on recording medium190and to carry reflected light340from recording medium190back to beam splitter330for delivery to sensor350. As in all the embodiments described herein, the optical fiber may advantageously be a single-mode optical fiber.

FIGS. 4A-4Dare graphs depicting various irradiance distributions of laser light. Irradiance (I) is plotted in the vertical direction vs. linear distance (y) from the center of each beam, plotted in the horizontal direction.FIG. 4Ashows a conventional Gaussian irradiance distribution400that is normally formed when the output beam of a single laser is focused on a recording medium. The horizontal dashed line410represents a threshold of irradiance for recording. Irradiance values less than410are not effective in recording on the recording medium. The vertical dashed lines420and430represent the distances from the beam center that irradiance falls below threshold410. InFIG. 4A, only the portion460above line410and between lines420and430is effective. Thus, energy in the portions of the distribution outside the region460, denoted by reference numerals440and450, is wasted.

When laser beams from distinct lasers, such as lasers110,120, and130ofFIG. 1or lasers305and310ofFIG. 3, are focused onto recording medium190from directions not coinciding with the central optical axis of hybrid optical element170or210, the beams may still be made to focus at nearly the same focal spot195on recording medium190, but their individual irradiance distributions at that focal spot (curves470and480), as shown inFIGS. 4B and 4C, may not be symmetric Gaussian distributions, i.e., they may be distorted as shown. The combined irradiance490is shown inFIG. 4D(normalized toFIGS. 4A,4B, and4C). While the combined irradiance may have more energy outside the effective region than inFIG. 4A(outside lines420and430), the energy from the combined power of two or more lasers in the central peak of this combined irradiance more than compensates for that deficiency.

The various embodiments of an optical print head disclosed herein, by including a number of lasers having laser emissions within a desired wavelength range, provide higher power at lower cost for monochromatic recording or provide for color optical recording by incorporating multiple wavelengths in the same optical print head. The optical fiber (e.g., in the form of a single-mode photonic crystal optical fiber) receiving combined light from the lasers at one end and emitting combined output light at its other end, combines the various laser emissions efficiently and allows separation of the heat-producing lasers from that portion of the print head adjacent to the recording medium. That portion may thus be made smaller and lighter than in an optical print head with lasers near the recording medium. The hybrid optical element of these embodiments, optically coupled to the exit end of the optical fiber and focusing the combined output light on the recording medium, provides efficient and low-cost coupling of laser light to the recording medium.

INDUSTRIAL APPLICABILITY

Devices made in accordance with the disclosed embodiments and their equivalents are useful in optical recording. Optical print head embodiments having laser light sources incorporating multiple lasers including various wavelengths are useful in color optical recording. Optical print head embodiments having laser light sources incorporating multiple lasers of the same wavelength are useful in optical recording at relatively high power. Optical print head embodiments employing an optical fiber may be used when separation of lasers from other components is required to avoid thermal interactions.

Although the foregoing has been a description and illustration of specific embodiments of the invention, various modifications and changes thereto can be made by persons skilled in the art without departing from the scope and spirit of the invention as defined by the following claims. For example, various equivalent materials or optical elements may be substituted for those described herein. For another example, hybrid optical element170may include an electrohologram for electronic control of focal length, NA, or other optical parameter.