DEVICE AND METHOD FOR OPTICAL PACKAGE FOR PROVIDING FEEDBACK

Abstract: The present invention provides a device (200, 300) and a method (100) for providing optical feedback for an optical package. The device includes: an optical emitter (202), responsive to an electrical input current (208), for generating a beam of light; a partially reflective holographic optical element/reflector (204), arranged to allow a first predetermined portion of the beam to exit the partially reflective holographic optical element/reflector and to diffract a second predetermined portion of the beam to an optical sensing unit (206); and the optical sensing unit, arranged to receive the second predetermined portion of the beam, for generating feedback for adjusting the electrical input current to the optical emitter based on a received optical power of the second predetermined portion of the beam.

Description: DEVICE AND METHOD FOR OPTICAL PACKAGE FOR PROVIDING FEEDBACK Field of the Invention The present invention relates to providing optical feedback for applications requiring optical emitters and, more particularly, to using holograms as a feedback mechanism. Background of the Invention The present invention relates, in general to a vertical cavity surface emitting laser (VCSEL). A VCSEL is a semiconductor laser diode in which the laser oscillation and the optical emission occur in a direction normal to the pn junction plane. This device has many properties which are advantageous when compared to the more commonly used edge-emitting laser diodes. These properties include: low optical beam divergence, a circular optical output, and single longitudinal mode operation. In addition, VCSEL devices are manufactured in a manner which allows wafer level testing of individual devices. These properties make VCSELs attractive in applications such as optical data storage, data communication and laser scanners. In most systems utilizing semiconductor lasers, dynamic stabilization and control of the optical output power is required. In edge emitting laser diodes this is usually accomplished by allowing a portion of the light emitted by the laser diode, i.e., using light emitted from the laser diode's rear mirror for bias control, to illuminate an optical photodetector which produces a photocurrent proportional to the intensity of the light emitted by the laser diode. This optically generated photocurrent, in turn drives a feedback circuit which controls the laser bias current. However, due to the structure of the VCSEL, light emitted from the rear mirror is absorbed by the GaAs substrate. Thus, the system of using light emitted fom the rear mirror is unavailable fo bias control. Monolithic integration of the photodetector to detect laser light from either the top or bottom mirrors has been attempted, "Monolithic Integration of Photodetector With Vertical Cavity Surface Emitting Laser", G. Hasnain et al., Electronic Letters, Vol. 27, No. 18, Aug. 29,1991 , pp.1630- 1632. This implementation, while attractive, is complex and may be very costly to manufacture. Integration of a lateral detector has also been proposed, "Feedback Mechanism for Vertical Cavity Surface Emitting Lasers", End Tabatataie, U. S. Patent No. 5,285,466. This technique has the limitation of only detecting the emission of spontaneous light from the laser. This results in a photocurrent which is not proportional to the total optical power of the VCSEL. Thus, there is a need for a device and method for an optical package for efficiently providing automatic optical feedback for an optical emitter. Brief Description of the Drawings FIG. 1 is a graphic representation of one embodiment of steps of a method in accordance with the present invention. FIG. 2 is a flow chart showing one embodiment of a device in accordance with the present invention. FIG. 3 is a schematic representation of one embodiment of a device in accordance with the invention. Detailed Description of a Preferred Embodiment The present invention provides a feedback mechanism for automatic power control for an optical emitter, thus providing automatic adjustment of electrical input current for the optical emitter. This method improves the power stability of semiconductor laser diodes. The utilization of the feedback mechanism provides the advantage that the whole package containing the holographic optical element adapts the optical emitter without significant mechanical modification. The feedback current may be controlled by the diffraction efficiency of the holographic optical element because the reflecting power is determined by the design of the holographic optical element. The design includes the index modulation amplitude, thickness, and grating structure of the holographic film. In addition, the use of the holographic optical element allows control of the size of a beam sent to the optical sensing element, thus allowing one to choose a smaller detector than was possible with the prior art. The present invention uses a hologram to direct a predetermined portion of the light emitted from the top surface of the VCSEL onto a separate adjacent photodetector. Thus, the resulting photocurrent is proportional to the total optical power of the VCSEL. In addition, the hologram can be designed to reflect a predetermined percentage of the total optical output power. The use of the hologram can also be used to reduce the area of illumination on the optical sensing element. This may result in a reduction in the size of the required photodetector. FIG. 1 , numeral 100, is a graphic representation of one embodiment of steps of a method in accordance with the present invention. The method provides optical feedback for an optical package. The method includes the steps of: A) generating a beam of light using an optical emitter responsive to an electrical input current (102); B) directing a first predetermined portion of the beam, using a partially reflective holographic optical element/reflector, to exit the partially reflective holographic optical element/reflector and to diffract a second predetermined portion of the beam to an optical sensing unit (104); C) generating feedback for adjusting the electrical input current to the optical emitter based on a received optical power of the second predetermined portion of the beam using an optical sensing unit that is arranged to receive the second predetermined portion of the beam (106). The partially reflective holographic optical element/reflector may be selected to be one of: A) a partially reflective volume holographic optical element recorded in a holographic film; B) a diffractive optical element other than a partially reflective volume holographic optical element; and C) a partially reflecting surface tilted at a predetermined angle. The optical emitter is typically one of: A) an edge emitting laser; B) a vertical cavity surface-emitting laser; and C) a light-emitting diode. The optical sensing unit is generally at least a photodetector. FIG. 2, numeral 200, is a flow chart showing one embodiment of a device in accordance with the present invention. The device for an optical package provides optical feedback and includes an optical emitter (202), a partially reflective holographic optical element or reflector (204), and an optical sensing unit (206). The optical emitter (202) is responsive to an electrical input current, and generates a beam of light. The partially reflective holographic optical element/reflector (204) is arranged to allow a first predetermined portion of the beam to exit the partially reflective holographic optical element/reflector and to diffract a second predetermined portion of the beam to an optical sensing unit (206). The optical sensing unit (206) is arranged to receive the second predetermined portion of the beam, for generating feedback for adjusting the electrical input current to the optical emitter based on a received optical power of the second predetermined portion of the beam. In general, the optical sensing unit (206) provides the feedback current to a current control unit (208) that inputs en electrical input current to the optical emitter (202). As in the method, the partially reflective holographic optical element/reflector (202) is typically one of: A) a partially reflective volume holographic optical element recorded in a holographic film; B) a diffractive optical element other than a partially reflective volume holographic optical element; and C) a partially reflecting surface tilted at a predetermined angle. The optical emitter (204) is generally one of: A) an edge emitting laser; B) a vertical cavity surface- emitting laser; and C) a light-emitting diode. A photo detector is often utilized as the optical sensing unit (206). An optical storage unit may utilize the device of the present invention. Typical optical storage units are: a compact disk player; a digital video disk player; a compact disk read- only memory drive; a magneto-optical disk drive; and a laser disk player. FIG. 3, numeral 300, is a schematic representation of one embodiment of a device in accordance with the invention. In this embodiment the device is typically an enclosed unit having an optical emitter (302), a partially reflective holographic optical element or reflector (304) for partialy reflecting the laser beam to an optical sensing unit (306) and allowing transmittal of another predetermined portion of the laser beam outside the enclosed unit. The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope. We claim:

Claims: 1. A device for an optical package for providing optical feedback, comprising: 1 A) an optical emitter, responsive to an electrical input current, for generating a beam of light; 1 B) a partially reflective holographic optical element/reflector, arranged to allow a first predetermined portion of the beam to exit the partially reflective holographic optical element/reflector and to diffract a second predetermined portion of the beam to an optical sensing unit; 1 C) the optical sensing unit, arranged to receive the second predetermined portion of the beam, for generating feedback for adjusting the electrical input current to the optical emitter based on a received optical power of the second predetermined portion of the beam. 2. The device of claim 1 wherein at least one of 2A-2C: 2A) the partially reflective holographic optical element/reflector is one of 2A1 -2A3: 2A 1 ) a partially reflective volume holographic optical element recorded in a holographic film; 2A2) a diffractive optical element other than a partially reflective volume holographic optical element; and 2A3) a partially reflecting surface tilted at a predetermined angle; 2B) wherein the optical emitter is one of 2B1 -2B3: 2B1 ) an edge emitting laser; 2B2) a vertical cavity surface-emitting laser; and 2B3) a light-emitting diode; and 2C) the optical sensing unit is at least a photo detector. 3. A device for providing optical feedback for automatic power control for a vertical cavity surface-emitting laser in an optical package, comprising: 3A) an optical emitter, responsive to an electrical current, for generating a beam of light; 3B) a partially reflective holographic optical element, arranged to allow a first predetermined portion of the beam to exit the optical package and to diffract a second predetermined portion of the beam to a photo detector; 3C) the photo detector, arranged to receive the second predetermined portion of the beam, for generating a feedback electrical current based on a received optical power of the second predetermined portion of the beam. 4. A method for providing optical feedback for an optical package, comprising the steps of: 4A) generating a beam of light using an optical emitter responsive to an electrical input current; 4B) directing a first predetermined portion of the beam, using a partially reflective holographic optical element/reflector, to exit the partially reflective holographic optical element/reflector and to diffract a second predetermined portion of the beam to an optical sensing unit; 4C) generating feedback for adjusting the electrical input current to the optical emitter based on a received optical power of the second predetermined portion of the beam using an optical sensing unit that is arranged to receive the second predetermined portion of the beam. 5. The method of claim 4 wherein at least one of 5A-5C: 5A) the partially reflective holographic optical element/reflector is one of 5A1 -5A3: 5A1 ) a partially reflective volume holographic optical element recorded in a holographic film; 5A2) a diffractive optical element other than a partially reflective volume holographic optical element; and 5A3) a partially reflecting surface tilted at a predetermined angle; 5B) the optical emitter is one of 5B1 -5B3: 5B1 ) an edge emitting laser; 5B2) a vertical cavity surface-emitting laser; and 5B3) a light-emitting diode; and 5C) the optical sensing unit is at least a photo detector. 6. A method for providing optical feedback for automatic power control for a vertical cavity surface-emitting laser in an optical package, comprising: 6A) generating a beam of light using an optical emitter, responsive to an electrical input current; 6B) directing a first predetermined portion of the beam, using a partially reflective holographic optical element, to exit the partially reflective holographic optical element and to diffract a second predetermined portion of the beam to a photo detector; 6C) generating feedback for adjusting the electrical input current to the optical emitter based on a received optical power of the second predetermined portion of the beam using the photo detector that is arranged to receive the second predetermined portion of the beam . 7. A device for an optical package for providing optical feedback, comprising: 7A) an optical emitter, responsive to an electrical input current, for generating a beam of light; 7B) a partially reflective holographic optical element/reflector, arranged to allow a first predetermined portion of the beam to exit the partially reflective holographic optical element/reflector and to diffract a second predetermined portion of the beam to an optical sensing unit; 7C) the optical sensing unit, arranged to receive the second predetermined portion of the beam, for generating feedback for adjusting the electrical input current to the optical emitter based on a received optical power of the second predetermined portion of the beam. 8. A bar code scanner having a device for an optical package for providing optical feedback, wherein the device comprises: 8A) an optical emitter, responsive to an electrical input current, for generating a beam of light; 8B) a partially reflective holographic optical element/reflector, arranged to allow a first predetermined portion of the beam to exit the partially reflective holographic optical element/reflector and to diffract a second predetermined portion of the beam to an optical sensing unit; 8C) the optical sensing unit, arranged to receive the second predetermined portion of the beam, for generating feedback for adjusting the electrical input current to the optical emitter based on a received optical power of the second predetermined portion of the beam. 9. An optical communication link having a device for an optical package for providing optical feedback, wherein the device comprises: 9A) an optical emitter, responsive to an electrical input current, for generating a beam of light; 9B) a partially reflective holographic optical element/reflector, arranged to allow a first predetermined portion of the beam to exit the partially reflective holographic optical element/reflector and to diffract a second predetermined portion of the beam to an optical sensing unit; 9C) the optical sensing unit, arranged to receive the second predetermined portion of the beam, for generating feedback for adjusting the electrical input current to the optical emitter based on a received optical power of the second predetermined portion of the beam. 10. An optical storage unit having a device for an optical package for providing optical feedback, wherein the device comprises: 1 0A) an optical emitter, responsive to an electrical input current, for generating a beam of light; 10B) a partially reflective holographic optical element/reflector, arranged to allow a first predetermined portion of the beam to exit the partially reflective holographic optical element/reflector and to diffract a second predetermined portion of the beam to an optical sensing unit; 10C) the optical sensing unit, arranged to receive the second predetermined portion of the beam, for generating feedback for adjusting the electrical input current to the optical emitter based on a received optical power of the second predetermined portion of the beam; and where selected, 10D) wherein the optical storage unit is one of 10D1 - 10D5: 10D1 ) a compact disk player; 10D2) a digital video disk player; 10D3) a compact disk read-only memory drive; 10D4) a magneto-optical disk drive; and 10D5) a laser disk player.

Inventors: HARTON AUSTIN V, JIANG WENBIN, LEBBY MICHAEL, LUREY DANIEL M, MORIKUNI JAMES J, WYATT KARL, YEH JANG-HUN JAMES, HARTON, AUSTIN, V., JIANG, WENBIN, LEBBY, MICHAEL, LUREY, DANIEL, M., MORIKUNI, JAMES, J., WYATT, KARL, YEH, JANG-HUN, (JAMES)
Applicants: MOTOROLA INC, MOTOROLA INC.
Application year: 1997
Main classification: G01J1
IPC codes: G01J1, G06K7, H01L33, H01S5