Harmonically mode-locked laser

• US 5,274,659 A
• Filed: 03/30/1992
• Issued: 12/28/1993
• Est. Priority Date: 02/18/1992
• Status: Expired due to Term

• Current Assignee: American Telephone & Telegraph Company (AT&T, Inc.)
• Original Assignee: AT&T, Inc.
• Inventors: Harvey, George T., Mollenauer, Linn F.
• Primary Examiner(s): Davie, James W.
• Application Number:
  US07/860,526
• Time in Patent Office: 638 Days
• Field of Search: 372/94, 372/18, 372/20, 372/25, 372/26, 372/6, 372/92, 372/100, 372/34
• US Class Current: 372/94
• CPC Class Codes: H01S 3/06791 Fibre ring lasers fibre laser gyrometers G01C19/72
• CPC Class Codes: H01S 3/082 defining a plurality of resonators, e.g. for mode selection or suppression
• CPC Class Codes: H01S 3/1062 using a controlled passive interferometer, e.g. a Fabry-Perot etalon
• CPC Class Codes: H01S 3/1109 Active mode locking
• CPC Class Codes: H01S 3/1121 Harmonically mode locking lasers, e.g. modulation frequency equals multiple integers or a fraction of the resonator roundtrip time
• CPC Class Codes: H01S 3/1305 Feedback control systems
• CPC Class Codes: H01S 3/1396 by using two modes present, e.g. Zeeman splitting H01S3/1398 takes precedence

Abstract

In a harmonically mode-locked laser, first (33) and second (34) optical beams are derived from the laser optical path which are directed, respectively, through first and second optical filters (37) having optical pass-bands (40, 41) that are displaced in frequency but which intersect at approximately the center frequency (f_c) of operation of the laser. The free spectral range of a Fabry-Perot optical resonator (24) in the laser ring is deviated from a frequency exactly equal to the pulse repetition rate of the laser by a frequency (df) sufficient to permit changes in the length of the optical path to be manifested as changes in the wavelength of light transmitted along the optical path. Apparatus (39, 40, 42) for detecting the difference of intensity of light passing through the first and second filters can then be used to generate a signal proportional to the difference of intensity of such light for the purpose of adjusting, by length adjusting apparatus (43), the length of the optical path to compensate for spurious temperature-induced length variations.

Claims

1. A harmonically mode-locked ring laser comprising:

means defining a resonant optical path;

an active laser medium for emitting coherent light to be transmitted along the optical path;

means included in the optical path for causing the light to propagate along the path as a succession of pulses having a substantially uniform repetition rate at a sub-optic frequency and having a period substantially equal to the transit time of the light pulses in the optical path divided by an integer;

the optical path including a Fabry-Perot optical resonator having a free spectral range substantially equal to said pulse repetition rate;

means for deriving first and second optical beams from the optical path;

means for directing the first and second beams respectively through first and second optical filters;

the first and second filters having optical pass-bands that are displaced in frequency and which intersect at a frequency f_c ;

means for detecting the difference of intensity of light passing through the first and second optical filters and for using the difference to adjust the length of the optical path;

the free spectral range of the Fabry-Perot optical resonator deviating from a frequency exactly equal to the pulse repetition rate by a frequency sufficient to permit changes in the length of the optical path to be manifested as changes in wavelength of light transmitted along the optical path.

2. The laser of claim 1 wherein:

the resonant optical path has a plurality of resonant ring modes each separated by a frequency f₂ ;

and the free spectral range of the Fabry-Perot resonator deviates from a frequency exactly equal to the pulse repetition rate by a frequency df that is less than f₂.

3. The laser of claim 1 wherein:

the first and second filters comprise part of a single wedged etalon.

4. The laser of claim 3 wherein:

a central portion of the wedged etalon constitutes part of said optical path, a relatively thick portion of the etalon constitutes the first filter and a relatively thin portion of the etalon constitutes the second filter.

5. The laser of claim 4 further comprising:

means for moving the etalon transversely with respect to the optical path.

6. The laser of claim 5 wherein:

the first and second optical beams and the optical path all traverse the wedged etalon along substantially parallel paths.

7. The laser of claim 1 wherein:

the means for detecting light intensity differences and using the differences to adjust length comprises photodetectors in the paths of the first and second beams, the outputs of the photodetectors are connected to a differential amplifier, and the output of the differential amplifier is connected to apparatus for adjusting the length of the optical path.

8. The laser of claim 1 wherein:

the optical path defining means comprises an optical fiber;

and the means for adjusting the length of the optical path comprises means for selectively resistively heating part of the optical fiber.

9. The laser of claim 8 wherein:

said part of the optical fiber comprises a single-mode optical fiber coated with film of resistive material;

and the selective heating means comprises means for directing current through the resistive metal film.

10. The laser of claim 1 wherein:

the means for causing light to propagate as a succession of pulses comprises an optical modulator for modulating optical transmission in the optical path at a rate equal to the pulse repetition rate;

and the optical path describes a closed loop having a transit time substantially equal to an integral number of periods of said pulse repetition rate.

11. The laser of claim 1 wherein:

the active laser medium comprises an erbium doped single-mode optical fiber portion;

and the succession of pulses are reached sufficiently close to being of a hyperbolic secant squared shape to be used as a source of solitons.

12. An optical pulse generator comprising:

means for defining an optical transmission path;

an active laser medium included in the transmission path;

means for causing the active medium to emit coherent light of a predetermined frequency;

means included in the optical transmission path for causing the emitted light to propagate along the transmission path as a succession of pulses having a substantially uniform pulse repetition rate at a sub-optic frequency;

the length of the path being substantially equal to an integral number of periods of said pulse repetition rate; and

means for removing a portion of said pulses from said optical transmission path;

wherein the improvement is characterized in that;

said optical transmission path includes an optical resonator having equally spaced resonances, adjacent resonances being separated by a frequency substantially equal to said pulse repetition rate;

means are provided for deriving first and second optical beams from said optical path and for directing the first and second optical beams, respectively, through first and second optical filters having optical pass-bands that are displaced in frequency and which intersect at a frequency f_c ;

and means are provided for detecting the difference of intensity of light passing through the first and second optical filters and for using the difference to adjust the length of the optical transmission path.

13. The pulse generator of claim 12 further characterized in that:

the optical path is a closed-loop path, a major part of which is defined by a single-mode optical fiber.

14. The pulse generator of claim 12 further characterized in that:

the optical resonator is a Fabry-Perot resonator.

15. The pulse generator of claim 14 further characterized in that:

the optical transmission path has a plurality of resonant modes each separated by a frequency f₁ ;

and the Fabry-Perot resonator has a free spectral range that deviates from a frequency exactly equal to the pulse repetition rate by a frequency that is less than f₁.

16. The pulse generator of claim 12 further characterized in that:

the first and second filters comprise part of a single wedged etalon;

and means are included for moving the etalon transversely with respect to the optical transmission path.