THERMOMAGNETIC RECORDING AND MAGNETO-OPTIC PLAYBACK SYSTEM HAVING CONSTANT INTENSITY LASER BEAM CONTROL

• US 3,778,791 A
• Filed: 10/06/1971
• Issued: 12/11/1973
• Est. Priority Date: 10/06/1971
• Status: Expired due to Term

• Current Assignee: George W. Lewicki, John E. Guisinger
• Original Assignee: George W. Lewicki, John E. Guisinger
• Inventors: Lewicki, George W., Guisinger, John E.
• Primary Examiner(s): Konick, Bernard
• Assistant Examiner(s): Tupper, Robert S.
• Application Number:
  US05/187,143
• Time in Patent Office: 797 Days
• Field of Search: 179/1.2CH 340/174.1M 346/74MT 340/174YC 350/151 250/205,217,225,227
• US Class Current: 360/59
• CPC Class Codes: B23K 26/04 Automatically aligning, aiming or focusing the laser beam, e.g. using the back-scattered light
• CPC Class Codes: G02F 1/0327 Operation of the cell; Circuit arrangements G02F1/05 takes precedence
• CPC Class Codes: G02F 1/092 Operation of the cell; Circuit arrangements
• CPC Class Codes: G11B 11/10508 by modulating only the magnetic field at the transducer
• CPC Class Codes: G11B 11/10515 Reproducing compensating pit shift G11B11/1053
• CPC Class Codes: G11B 11/10536 using thermic beams, e.g. lasers
• CPC Class Codes: G11B 11/10543 using optical beam of radiation
• CPC Class Codes: G11B 11/10595 Control of operating function
• CPC Class Codes: G11B 7/126 Circuits, methods or arrangements for laser control or stabilisation

Abstract

Apparatus is disclosed for maintaining the intensity of a laser beam substantially constant in a thermomagnetic recording and magneto-optic playback system wherein an isotropic film is heated along a continuous path by the laser beam for recording. As each successive area of the path is heated locally to the vicinity of its Curie point in the presence of a controlled magnetic field, a magneto-optic density is produced proportional to the amplitude of the controlled magnetic field. To play back the recorded signal, the intensity of the laser beam is reduced and a Faraday or Kerr effect analyzer is employed, with a photodetector, as a transducer for producing an output signal. The light intensity of the laser beam is continuously detected close to the analyzer, particularly during playback operation, and compared with a reference to maintain intensity substantially constant.

Claims

1. In a magneto-optic playback system for producing an electrical signal proportional to the direction and amplitude of magnetization of discrete areas of a film, said system having means for directing a continuous laser beam through a polarizer to said film as said film is moved passed a playback station, whereby said discrete areas rotate said polarized light impinging thereon in proportion to the magnetic fields of said areas, said system further having analyzing means disposed to receive said laser beam after it has been rotated by said discrete areas for producing said electrical signal, apparatus for maintaining the intensity of said laser beam substantially constant comprising:

means for producing a continuous laser beam, said laser beam producing means including beam intensity control means responsive to a control signal;

means for continuously sampling said laser beam at a point in the beam path close to said analyzing means and for producing in response thereto a feedback signal proportional to the intensity of the beam sample;

a source of a substantially constant reference signal;

means for comparing said reference signal with said feedback signal to produce an error signal proportional to the difference in amplitude between said feed-back signal and said reference signal;

means for integrating said error signal to produce said control signal; and

means for applying said control signal to said beam intensity control means, thereby maintaining the intensity of said laser beam substantially constant.

2. Apparatus as defined in claim 1 wherein said means for producing a continuous laser beam comprises a gas laser, and said beam intensity control means comprises a device which exhibits an electro-optic effect in response to said control voltage signal to vary the intensity of said laser beam to said utilization means.

3. Apparatus as defined in claim 1 wherein said means for producing a continuous laser beam comprises an injection laser having a suitably prepared and forward biased semiconductor diode, and said beam intensity control means comprises means for controlling the level of forward bias of said diode.

4. In a magneto-optic playback system for producing an electrical output signal proportional to the direction and amplitude at all levels between two extremes of magnetization of discrete areas of a film as successive areas of said film are moved through a playback station, the combination comprising:

a gas laser for producing a beam of light;

an electro-optic device for controlling the intensity of light transmitted therethrough in response to a control signal;

a light polarizer;

means for directing said light beam through said electro-optic device and said light polarizer to said film at said playback station;

analyzing means including a resolver for receiving said beam of light, after it has impinged upon said film and has been rotated by the magnetic fields of successive areas, and producing said output signal;

means responsive to the intensity of unresolved light for producing a feedback signal;

means interposed in the light path between said film and said analyzing means for directing part of said light beam toward said analyzing means and directing the balance of said light beam toward said feedback signal means;

a source of a substantially constant reference signal;

means for comparing said reference signal with said feedback signal to produce an error signal proportional to the difference in amplitude between said feedback signal and said reference signal;

means for integrating said error signal to produce a control signal; and

means for applying said control signal to said electro-optic device to maintain the intensity of light transmitted therethrough substantially constant.

5. Apparatus as defined in claim 4 including apparatus for employing said magneto-optic playback system for thermomagnetic recording of an input signal in response to a record mode select signal comprising:

means at said playback station responsive to said record mode select signal for producing a magnetic field proportional to said input signal to be recorded; and

variable gain control means responsive to said record mode select signal to decrease said feedback signal, thereby increasing the intensity of said beam of light for thermomagnetic recording.

6. Apparatus as defined in claim 4 wherein said reference signal is a square wave and said comparing means comprises:

means responsive to said reference signal for modulating said feedback signal to produce a modulated feedback signal;

a first amplifying means coupling half cycles of said reference signal of a given polarity to said integrating means; and

a second amplifying means coupling half cycles of said modulated feedback signal of a polarity opposite said given polarity to said integrating means, whereby said modulated feedback signal applied to said second amplifying means need not by synchronized with said reference signal applied to said first amplifying means.

7. In a magneto-optic system for producing an electrical output signal proportional to the direction and amplitude at all levels between two extremes of magnetization of discrete areas of a film as successive areas of said film are moved through a playback station, the combination comprising:

an injection lAser for producing a beam of light, said injection laser having a suitably prepared semiconductor diode forward biased by a signal controlled bias means;

a light polarizer;

means for directing said light beam through said polarizer to said film at said playback station;

analyzing means including a resolver for receiving said beam of light, after it has impinged said film and has been rotated by the magnetic fields of successive areas, and producing said output signal;

means responsive to the intensity of unresolved light for producing a feedback signal;

means interposed in the light path between said film and said analyzing means for directing part of said light beam toward said analyzing means and directing the balance of said light beam toward said feedback signal means;

a source of a substantially constant reference signal;

means for comparing said reference signal with said feedback signal to produce an error signal proportional to the difference in amplitude between said feedback signal and said reference signal;

means for integrating said error signal to produce a control signal; and

means for applying said control signal to said signal controlled bias means to maintain the intensity of said light beam substantially constant.

8. Apparatus as defined in claim 7 including apparatus for employing said magneto-optic playback system for thermomagnetic recording of an input signal in response to a record mode select signal comprising:

means at said playback station responsive to said record mode select signal for producing a magnetic field proportional to said input signal to be recorded; and

variable gain control means responsive to said record mode select signal to decrease said feedback signal, thereby increasing the intensity of said beam of light for thermomagnetic recording.

9. Apparatus as defined in claim 7 wherein said reference signal is a square wave and said comparing means comprises:

means responsive to said reference signal for modulating said feedback signal to produce a modulated feedback signal;

a first amplifying means coupling half cycles of said reference signal of a given polarity to said integrating means; and

a second amplifying means coupling half cycles of said modulated feedback signal of a polarity opposite said given polarity to said integrating means, whereby said modulated feedback signal applied to said second amplifying means need not be synchronized with said reference signal applied to said first amplifying means.