Scanning temporal ultrafast delay methods and apparatuses therefor

• US 6,396,856 B1
• Filed: 03/27/1998
• Issued: 05/28/2002
• Est. Priority Date: 04/01/1994
• Status: Expired due to Term

• Current Assignee: IRMA America Incorporated
• Original Assignee: IRMA America Incorporated
• Inventors: Harter, Donald J., Fermann, Martin E., Sucha, Gregg D.
• Primary Examiner(s): Davie, James W.
• Assistant Examiner(s): ZAHN, JEFFREY N
• Application Number:
  US09/169,577
• Time in Patent Office: 1,523 Days
• Field of Search: 372/20,25,30
• US Class Current: 372/25
• CPC Class Codes: G01P 3/806 in devices of the type to be classified in G01P3/68
• CPC Class Codes: G01R 13/347 using electro-optic elements
• CPC Class Codes: G01S 17/18 wherein range gates are used
• CPC Class Codes: G01S 17/50 Systems of measurement based on relative movement of target
• CPC Class Codes: G01S 7/484 Transmitters
• CPC Class Codes: G01S 7/486 Receivers
• CPC Class Codes: G01S 7/497 Means for monitoring or calibrating
• CPC Class Codes: H01S 3/1398 by using a supplementary modulation of the output
• CPC Class Codes: H01S 3/2383 Parallel arrangements

Abstract

The present invention is directed to methods and apparatuses for performing temporal scanning using ultra-short pulsewidth lasers in which only minimal (micro-scale) mechanical movement is required. The invention also relates to methods for obtaining high-accuracy timing calibration, on the order of femtoseconds. A dual laser system is disclosed in which the cavity of one or more of the lasers is dithered, by using a piezoelectric element. A Fabry-Perot etalon is used to generate a sequence of timing pulses used in conjunction with a laser beam produced by the laser having the dithered laser cavity. A correlator correlates a laser pulse from one of the lasers with the sequence of timing pulses to produce a calibrated time scale. The methods and apparatuses of the present invention are applicable to many applications requiring rapid scanning and time calibration, including, but not limited to metrology, characterization of charge dynamics in semiconductors, electro-optic testing of ultrafast electronic and optoelectronic devices, optical time domain reflectometry, and electro-optic sampling oscilloscopes.

Claims

1. A fast scanning laser apparatus, comprising:

a controllable source of electrical signals which outputs a dithering signal; and

first and second short-pulse laser sources each having a laser cavity, wherein at least one of said first and second laser sources has a length changing unit cooperating with its cavity, said length changing unit changing the length of said laser cavity based on said dithering signal.

2. The fast scanning apparatus as claimed in claim 1, further comprising:

a stabilizer which generates a control signal based on timing differences between a first pulse output from said first laser source and a second pulse output from said second laser source, and which outputs said control signal to said length changing unit which thereby changes the length of said laser cavity based on said control signal and said dithering signal.

3. The fast scanning apparatus as claimed in claim 2, wherein said stabilizer comprises:

a phase lock loop circuit for detecting and controlling a phase difference between a first pulse output from said first short-pulse laser source and a second pulse output from said second short-pulse laser source;

means for generating the control signal based on the phase difference detected by said phase lock loop circuit, and adjusting and outputting said control signal.

4. The fast scanning apparatus as claimed in claim 1, wherein said length changing unit comprises a mirror mounted on a piezoelectric device.

5. The fast scanning laser apparatus as claimed in claim 1, wherein the length changing unit changing the length of the laser cavity thereby changes a repetition rate of the laser pulses of the short-pulse laser source having the length changing unit.

6. The fast scanning laser apparatus claimed in claim 1, wherein a mirror is mounted on said length changing unit, and said length changing unit causes said mirror to move which causes an effective length of the laser source having the length changing unit to be changed thereby changing a repetition rate of a laser beam output from the laser source having the length changing unit.

7. The fast scanning apparatus claimed in claim 1, wherein said length changing unit is a drum having an changeable diameter, and the laser source having the length changing unit including a fiber wrapped on said length changing unit, wherein when said diameter of said drum changes an effective length of the laser source having the length changing unit changes thereby changing a repetition rate of the laser beam output from the laser source having the length changing unit.

8. The fast scanning apparatus as claimed in claim 7, wherein said length changing unit is a piezoelectric drum.

9. The fast scanning laser apparatus as claimed in claim 1, wherein said first and second short-pulse laser sources output first and second ultrashort pulses, respectively, the apparatus further comprising:

an optical element for producing a sequence of pulses from said second pulse;

a timing unit for correlating said first pulse with said sequence of pulses produced by said optical element and outputting a calibration signal indicating a timing delay of said second pulse with respect to said sequence of pulses;

an optical output device for outputting said first pulse to an object under test, and receiving a reflected pulse corresponding to a reflection of said first pulse;

a correlator for correlating said reflected pulse with said second pulse and outputting a correlated signal; and

a data acquisition unit for comparing said correlated signal with said calibration signal.

10. The fast scanning laser apparatus as claimed in claim 9, wherein the apparatus is employed in a metrology system for measuring a distance, said object under test is a surface, and said optical output device is a lens for focussing said first pulse onto said surface.

11. The fast scanning laser apparatus as claimed in claim 9, wherein the apparatus is an optical time domain reflectometer, wherein said object under test is an optical fiber or other optical component attached thereto and said reflections are reflections of said first pulse from optical discontinuities within said optical fiber or said optical component or between said optical fiber and said optical component.

12. The fast scanning laser apparatus as claimed in claim 1, wherein said first and second short-pulse laser sources output first and second ultrashort pulses, respectively, the apparatus further comprising:

an optical element for producing a sequence of pulses from said first pulse;

an optical output device for outputting said first pulse to an object under test, and receiving a reflected pulse corresponding to a reflection of said first pulse by said object; and

a correlator for correlating said second pulse with said reflected pulse and with said sequence of pulses produced by said optical element, said correlator outputting a signal corresponding to the correlation of said second pulse with said reflected pulse which is superimposed on said sequence of pulses.

13. A measurement apparatus including the fast scanning laser apparatus as claimed in claim 1, wherein the measurement apparatus is an electro-optic sampling or a photoconductive sampling oscilloscope, and said first and second short-pulse laser sources output first and second ultrashort pulses, respectively, the apparatus further comprising:

an optical element for producing a sequence of pulses from said second pulse;

a timing unit for correlating said first pulse with said sequence of pulses produced by said optical element and outputting a calibration signal indicating a timing delay of said second pulse with respect to said sequence of pulses;

an optical output device for outputting said first pulse to a device under test, thereby enabling a signal on said object under test;

a probe for outputting said second pulse to a point to be measured on said device under test, and receiving a returned signal, wherein said returned signal is altered by an interaction between said enabled signal on said device under test and said second pulse output by said probe; and

a data acquisition unit for comparing said correlated signal with said returned signal, thereby providing a high precision time base.

14. An apparatus as claimed in claim 1, comprising:

means for isolating at least one of said first and second short-pulse laser sources from an external environment, wherein the first and second short-pulse laser sources are fiber lasers, comprising a fiber spool onto which the fiber laser is wound.

15. The apparatus as claimed in claim 14, wherein the thermal expansion of said fiber spool is matched to that of said optical fiber.

16. The apparatus as claimed in claim 14, wherein the isolated short-pulse laser source is the first short-pulse laser source and the stability of the second short-pulse laser source is controlled along with that of the first short-pulse laser source, by means of at least one stabilizer means selected from the group consisting of:

a construction whereby the first and second short-pulse laser sources are constructed from identical components in an identical fashion;

a shared laser for pumping the first and second short-pulse laser sources;

a shared spool onto which the first and second short-pulse laser sources are placed.

17. The apparatus as claimed in claim 14, further comprising:

an enclosure into which the first and second short-pulse laser sources are placed said enclosure being at least one of acoustically damped and temperature controlled.

18. The apparatus claimed in claim 9, wherein said optical element is selected from the group of elements consisting of:

a solid optical etalon;

an air-spaced optical etalon;

an optical etalon formed in an optical fiber cavity with end reflectors comprised of one of dielectric mirrors and photo-refractive fiber gratings;

an optical resonant reflector;

an optical fiber with misaligned splices to produce reflections;

an optical fiber with a plurality of photorefractively grown fiber gratings having a predetermined reflectivity and spacing;

an optical fiber with micro-bends placed to produce reflections at desired time intervals;

a semiconductor laser diode biased near a threshold to produce repetitive pulses, thereby preventing or retarding the decay of an optical pulse train;

a linear Fabry-Perot cavity containing optical fiber gain medium pumped near a threshold to produce repetitive pulses, thereby preventing or retarding the decay of an optical pulse train;

a passive optical fiber loop for storage of one or more pulses, said fiber loop being injected and dumped by an optical switching element;

an amplifying optical fiber loop for storage of one or more pulses, said fiber loop being injected and dumped by an optical switching element; and

a multimode optical fiber where modal dispersion creates different arrival times for pulses traveling in different transverse modes.

19. The apparatus as claimed in claim 9, wherein said optical element is a pulse train generator comprising:

a first optical fiber having a plurality of photorefractively-grown chirped fiber gratings chirped in a first direction;

a second optical fiber having a plurality of photorefractively-grown chirped fiber gratings, substantially the same as in the first optical fiber, chirped in a second direction which is opposite to said first direction; and

means for directing one of the first and second ultrashort pulses to one of said first and second optical fibers and receiving a first reflected plurality of pulses therefrom, and directing said first reflected plurality of pulses to the other of said first and second optical fibers and receiving a second reflected plurality of pulses therefrom, and outputting said second reflected plurality of pulses as a calibrated time scale.

20. The fast scanning laser apparatus as claimed in claim 1, further comprising:

a detector for detecting a temporal difference between the first and second laser pulses, wherein the controllable source of electrical signals generates the dithering signal; and

a stabilizer unit for generating a control signal based on the detected temporal difference,

wherein the length changing unit adjusts the output of said second short-pulse laser based on said control signal, and another length changing unit simultaneously adjusts the output of said first short-pulse laser, based on said dithering signal.

21. The fast scanning laser apparatus as claimed in claim 1, further comprising:

a detector for detecting a temporal difference between first and second laser pulses output from the first and second short-pulse laser sources, respectively, wherein the controllable source of electrical signals generates the dithering signal; and

a stabilizer unit for generating a control signal based on the detected temporal difference,

wherein the length changing unit adjusts the output of said second short-pulse laser source based on a combination of said control signal and said dithering signal.

22. The apparatus as claimed in claim 14, wherein said fiber spool is acoustically damped.