Apparatus for amplitude modulation of laser light

Abstract: Using the interferometer technique, a laser light beam is split into two beams one of which is directed along a first light path having a feed length and the other is directed along a second light path having a variable length. The length of the second light path is varied by a mirror assembly mounted on a piezoelectric block, the length of which is controlled by a modulation signal corresponding to the desired information signal. When the two light beams are recombined at the ends of the first and the second light paths, the varying of the length of the second light path results in a variable phase of the second light beam with respect to the first light beam that is modulation signal dependent at the precise point of recombination of the first and second light beams. The effect of the variable phase combination of the first and second light beams is a vectorial phase addition, the resultant laser light beam after this vectorial combination being intensity (amplitude) modulated by the modulation signal.

Description: The subject invention relates to an apparatus for amplitude modulation of a laser beam with transmission signal information comprising : a laser (10) for providing a laser light beam (12); a beam splitter (14) for splitting said laser beam into a first light beam (16) and a second light beam (18); a signal source for providing information signals (I) to be transmitted by said laser beam; a mirror assembly (20) arranged in the path (C) of said second light beam and coupled to a block of piezoelectric material (24) for displacing said mirror assembly, thereby varying the length of said second light beam path (C); electronic means (32) having an input coupled to said signal source and an output coupled to said block of piezo-electric material such that the length of said second light path is varied in accordance with the information signal; a beam combiner (14) for recombining said first and second light beams at the ends of the first light beam path (A) and the second light beam path (C) whereby the varying of the length of the second light path results in an amplitude modulation of the combined laser beam (22). US.Pat.3.499.699 discloses a wide band light modulator comprising a laser source and a beam splitter for splitting the beam from the laser source into a first light beam and a second light beam. In the path of each of these beams a first, low frequency, modulator formed by a movable reflector and a second, high frequency, modulator in the form of an electro-optic cell are present. After the first and second beam have been reflected by their respective reflector they are recombined by the beam splitter. The amplitude of the combined beam is determined by the optical path length variation for both beams introduced by their respective reflector and electro-optic cell. The object of the present invention is to provide a light modulating apparatus having a simpler construction and showing a high degree of linearity and stability. This apparatus is characterized in that, in order to compensate for thermal expansion of said block of piezoelectric material (24), said block is attached to the center of a floating intermediate plane (26), the opposite ends of said plane being attached to a fixed reference plane (27) at both sides of said first block via two further blocks of piezoelectric material (26,28) having substantially the same size and mass as said first block. With the above and additional objects and advantages in mind as will hereinafter appear, the invention will be described with reference to the accompanying drawing, in which the sole Figure shows a block schematic diagram of the invention. As shown in the Figure, a laser light source 10 generates a laser light beam 12 which is directed along light path A. A beam splitter 14 splits the laser light beam 12 into a first light beam 16, which travels horizontally through the beam splitter 14 along a light path B, and a second light beam 18, which travels vertically downward away from the beam splitter 14 along a light path C. In order to achieve maximum modulation, it is preferable that the first and second light beams 16 and 18 have the same intensity. Light path C includes a mirror arrangement 20 which reflects the second light beam 18 back to the beam splitter 14 where it is recombined with the first light beam 16. The resultant laser light beam 22 then travels along light path D. In order to vary the length of the light path C, the mirror arrangement 20 is mounted to an end of an actuator in the form of a block 24 of piezoelectric material, the opposite end thereof being mounted in the center of a intermediate base plane 26. The opposite ends of the intermediate base plane 26 are then attached to a fixed reference plane 27 by two blocks 28 and 30 of piezoelectric material. The three blocks 24, 28 and 30 are the same size and have equal mass. An information signal source (not shown) provides information signals I for modulating the laser light beam. These signals, which may be analog or digital, are applied through a wide band amplifier 32 to the block 24 resulting in the length thereof being varied through electrostrictive motion. It is important that the electrostrictive motion of block 24 be limited to no more than 1/2 the wavelength of the laser light. The blocks 28 and 30 are provided to compensate for any thermal expansion of block 24 and to provide electrostrictive "fine tuning" of the length of the light path C. To this end, the recombined laser light beam 22 in the light path D is applied to a second beam splitter 34 forming a third light beam 36 which travels along light path E, and a fourth light beam 38 which travels vertically along light path F. This fourth light beam 38 is applied to an optical detector 40 which is coupled to a sample gate and correction control circuit 42. The electrical energy derived from the optical detector 40 is sampled only during those times when an information signal is not present. Hence, the sample gate and correction control circuit 42 has the input information signal applied thereto as a control signal. The sample gate and correction control circuit 42 includes a memory for storing a previous sample and if the present sample is substantially different from the stored sample, indicative of thermal expansion of the block 24, a D.C. correction voltage is applied, through sample-and-hold circuit 44, to the blocks 28 and 30. This same procedure is also used to electrostrictively fine tune the quiescent length light path C by adjusting for maximum signal output or null, whichever is desired.

Claims: An apparatus for amplitude modulating a laser beam with transmission signal information comprising : a laser (10) for providing a laser light beam (12); a beam splitter (14) for splitting said laser beam into a first light beam (16) and a second light beam (18); a signal source for providing information signals (I) to be transmitted by said laser beam; a mirror assembly (20) arranged in the path (C) of said second light beam and coupled to a block of piezoelectric material (24) for displacing said mirror assembly, thereby varying the length of said second light beam path (C); electronic means (32) having an input coupled to said signal source and an output coupled to said block of piezo-electric material such that the length of said second light path is varied in accordance with the information signal; a beam combiner (14) for recombining said first and second light beams at the ends of the first light beam path (A) and the second light beam path (C) whereby the varying of the length of the second light path results in an amplitude modulation of the combined laser beam (22), characterized in that, in order to compensate for thermal expansion of said block of piezoelectric material (24), said block is attached to the center of a floating intermediate plane (26), the opposite ends of said plane being attached to a fixed reference plane (27) at both sides of said first block via two further blocks of piezoelectric material (26,28) having substantially the same size and mass as said first block. An apparatus as claimed in claim 1, characterized in that, for fine tuning of said second light path, it comprises: a second beam splitter (34) for splitting the amplitude modulated laser beam (22) into a third outgoing light beam and a fourth light beam (38); an optical detector (40) for detecting the fourth light beam; and an electronic correction circuit (42,44) receiving the output from said optical detector and generating a correction signal for said further blocks (28,30) in response to a difference between said output and a previously stored signal. An apparatus as claimed in claim 2, characterized in that the correction circuit is only operative when said information signals (I) are not present. An apparatus as claimed in claim 1 or 2, characterized in that said first light beam and said second light beam (18) are of equal intensity.

Inventors: BOARDMAN ALLEN H, KAYLOR WILLIAM ROBERT, BOARDMAN, ALLEN, KAYLOR, WILLIAM ROBERT
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 1993
Main classification: G02B26
IPC codes: G02B26, H01S3