Electro-optical light modulator driven by a resonant electrical circuit

• US 5,189,547 A
• Filed: 05/28/1991
• Issued: 02/23/1993
• Est. Priority Date: 05/28/1991
• Status: Expired due to Term

• Current Assignee: Newport Corporation (MKS Instruments Incorporated)
• Original Assignee: New Focus Inc. (MKS Instruments Incorporated)
• Inventors: Day, Timothy, Marsland, Robert A. Jr.
• Primary Examiner(s): LaRoche, Eugene R.
• Assistant Examiner(s): Lester, Evelyn A.
• Application Number:
  US07/706,292
• Time in Patent Office: 637 Days
• Field of Search: 332/139, 332/144, 332/147, 332/148, 359/245, 359/315, 359/84, 359/85, 359/279, 359/181, 359/183, 359/162, 359/163, 359/165, 359/198, 359/254, 359/264, 310/316, 310/320, 310/321
• US Class Current: 359/245
• CPC Class Codes: G02F 1/0327 Operation of the cell; Circuit arrangements G02F1/05 takes precedence

Abstract

A crystal exhibiting a variable index of refraction in response to a voltage applied across it is driven from a lower voltage signal generator through a passive resonant circuit. The resonant frequency of the circuit is tunable by hand in order to maximize the voltage applied to the crystal from a low voltage signal generator. The impedance of the circuit is also hand adjustable in order to match that of the signal generator. The crystal and driving circuit are packaged together in a single enclosure having optically transparent windows allowing a laser beam to pass through the crystal and be modulated by it according to the frequency of the signal generator.

Claims

1. A driving circuit connected to an optical phase modulating crystal that presents a capacitive load to the circuit, comprising:

a transformer having a primary winding and a secondary winding,

means connected to one terminal of the primary winding for providing a first manually selectable capacitance in series with the primary winding and a source of a radio frequency signal,

means connected as part of a series circuit with said transformer secondary winding and said crystal for providing a manually selectable inductance therein, and

means connected in parallel with said crystal for providing a second manually selectable capacitance thereacross,

whereby an overall impedance and resonant frequency of the circuit is controllable over a limited range by selection of the inductance, first capacitance and second capacitance values.

2. The driving circuit according to claim 1 wherein said transformer is characterized by having a number of turns of its said primary winding that is significantly more than a number of turns of its said secondary winding.

3. A driving circuit connected to an optical phase modulating crystal that presents a capacitive load to the circuit, comprising:

a transformer having a primary winding adapted to be connected to a radio frequency signal source, a secondary winding adapted for connection to said crystal, and means for allowing adjustment of an amount of coupling between said primary and secondary windings, and

means connected across said transformer secondary winding for providing a manually selectable inductance thereacross,

whereby an overall impedance and resonant frequency of the circuit is controllable over a limited range by adjustment of the transformer coupling adjustment means and the selectable inductance.

4. An electro-optical modulator, comprising:

an enclosure having a pair of windows on opposite sides thereof along an optical axis passing therethrough,

a crystal positioned within the enclosure along said optical axis and characterized by being substantially transparent with a refractive index that varies in proportion to an amount of voltage that is applied across said crystal at driving terminals thereof and which presents a capacitive load across said driving terminals,

an electrical connector carried by said enclosure in a manner allowing connection with a signal generator outside of said enclosure,

a printed circuit board positioned within said enclosure and connected with said crystal and said signal generator connector, and

a passing resonant driving circuit carried by said printed circuit board and interconnecting said crystal driving terminals and said electrical connector in a manner to provide a voltage across the crystal that is much greater than that received at said connector from a signal generator, said driving circuit including means providing at least one manually adjustable inductor for tuning the resonant frequency of said driving circuit.

5. The electro-optical modulator according to claim 4 wherein said tuning means of said driving circuit is connected in series with the crystal and a secondary winding of a transformer provided on said circuit board, a primary winding of said transformer being connected with said electrical connector.

6. The electro-optical modulator according to claim 5 wherein said driving circuit additionally includes means connected across said crystal terminals for providing a manually selectable level of capacitance thereacross.

7. The electro-optical modulator according to claim 6 wherein said driving circuit additionally includes means including a manually selectable capacitance value connected in series with the transformer primary winding and said electrical connector to provide a fine adjustment of the impedance and resonant frequency of the driving circuit at said electrical connector.

8. The electro-optical modulator according to claim 4 wherein said tuning means of said driving circuit is connected across said crystal terminals electrically parallel with said crystal.

9. A method of electrically driving an optical phase modulating crystal from a source of a radio frequency signal of a given frequency, comprising the steps of:

providing a transformer having primary and secondary winding terminals,

connecting said crystal to the transformer secondary winding terminals in series with an inductance and with a first capacitance connected in parallel with said crystal,

connecting the radio frequency signal source to the transformer primary winding terminals through a second capacitance,

adjusting values of said inductance and first capacitance until the combination of the series circuit and the first capacitance exhibits a resonant frequency substantially equal to said given frequency,

thereafter adjusting the second capacitance to substantially match an impedance of the radio frequency source, wherein the resonant frequency of the series circuit and the first capacitance is caused to move somewhat from said given frequency, and

thereafter further adjusting said inductance and said first capacitance in a manner to restore the resonant frequency of the series circuit and first capacitance substantially to said given frequency, whereby the resonant frequency and impedance are substantially optimized.