Scanning temporal ultrafast delay methods and apparatuses...

Details Scanning temporal ultrafast delay methods and apparatuses... Scanning temporal ultrafast delay methods and apparatuses...

1998-03-27

2002-05-28

Davie, James W. (Department: 2881)

Coherent light generators

Particular beam control device

Control of pulse characteristics

C372S030000

Reexamination Certificate

active

06396856

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention is related to the field of ultra-short pulsewidth lasers, and particularly to apparatuses and methods for performing temporal scanning with minimal (i.e., micron-scale) mechanical movement. The invention also relates to methods used for obtaining high-accuracy (i.e., sub-picosecond) timing calibration, applicable to the above-mentioned temporal scanning methods or to conventional temporal scanning methods. In particular, the invention eliminates the need for a mechanical scanning delay arm in a correlator or other type of pump-probe device, including ranging, 3-D imaging, contouring, tomography, and optical time-domain reflectometry (OTDR).
2. Description of the Related Art
Ultrafast laser oscillators are presently known which are capable of generating pulsewidths of the order of tens of femtoseconds with nanojoule-level pulse energies, at repetition rates ranging from 5 MHz to as high as 1 GHz. Such short pulses are used for many applications including measurements by time gating, including metrology. Many applications of such short optical pulses require that one set of optical pulses be delayed with respect to another set of optical pulses, in which temporal delays must be known to a very high accuracy, such as on the order of 10 femtoseconds. Temporal delays for short pulses find many uses in such applications as biological and medical imaging, fast photodetection and optical sampling, optical time domain reflectometers, and metrology.
The conventional method for delaying and scanning optical pulses is to reflect the pulses from a mirror and to physically move the mirror, using some mechanical means, by a distance D, which is defined by the product of the time delay, &Dgr;T, and the speed of light in vacuum c=3.0×10
8
meters/sec. Thus:
D=c/
2×&Dgr;
T
or
D
(cm)=15×&Dgr;
T
(nsec).
This type of delay will be termed here a physical delay. Also, scanning, as that term is used here, refers to the systematic changing of the difference in time of arrival between two optical pulses. Various methods and devices have been developed to provide the accurate positioning and scanning of the mirror, involving:
Voice-coil type devices (shakers) (R. F. Fork Nd F. A. Beisser,
APPL Opt.
17, 3534 (1978)).
Rotating mirror pairs (Z. A. Yasa and N. M. Amer,
Opt. Comm.,
36, 406 (1981)).
Linear translators employing stepper motors, which are commercially available from many vendors.
Linear translators employing galvanometers. (D. C. Edelstein, R. B. Romney, and M. Scheuermann,
Rev. Sci, Instrum.
62, 579 (1990)).
Other types of physical delays use adjustable group delay including:
Femtosecond pulse shapers (FPS) employing scanning galvanometers (K. F. Kwong, D. Yankelevich, K. C. Chu, J. P. Heritage, and A. Dienes; “400-Hz mechanical scanning optical delay line”
Opt. Lett.
18, (7) 558 (1993) (hereinafter Kwong et al.); K. C. Chu, K. Liu, J. P. Heritage, A. Dienes,
Conference on Lasers and Electro
-
Optics, OSA Tech.Digest Series, Vol.
8, 1994, paper CThI23.).
Rotating glass blocks.
The physical delay methods suffer from a number of disadvantages, the chief one being the large space required if large delays are desired. For example, a delay of 10 nsec requires a mirror displacement of 5 feet. There are other physical limitations and disadvantages as well. Misalignment and defocusing can distort measurements when large delays are used. Using corner-cube retroreflectors reduces the problem of misalignment, but not defocusing. The defocusing effect can occur when the scan amplitude is an appreciable fraction of the confocal parameter of the beam. A time delay of 10 nsec entails a change in free-space propagation of 10 feet (~3 meters). Thus, to minimize the effects of defocusing, the confocal parameter (Z
R
) must be approximately 10-times this number or Z
R
=30 meters. At a wavelength of 1550 nm, this requires a beam radius of w
o
=12 mm. This is impractically large for many situations.
The need for large mirror displacement can be reduced by multi-passing the delay line (e.g., double-passing the delay line cuts the required mirror displacement in half), however, this does not alleviate the defocusing problem. Multipassing causes its own set of problems in that alignment procedures are more complicated, and the optical losses are increased.
Yet another limitation has to do with the scanning rates and scanning frequencies which can be achieved simultaneously. It is often desirable to signal average while scanning rapidly (>30 Hz) in order to provide “real-time” displays of the measurement in progress. However, the scanning range is limited at such high scan frequencies. The best achieved scan range is 100 psec at a rate of 100 Hz using the scanning FPS method (Kwong et al.). Any further increase of the scanning range and/or frequency with such reciprocating devices would cause high levels of vibration, which can be disruptive to laser operation. Rotating glass blocks avert the vibration problem, and are capable of higher scan speed, but lack any adjustability of scan range, and they introduce variable group velocity dispersion which makes them inappropriate for use with pulses having widths shorter than 100 fsec.
In addition to the physical delays, methods have been introduced which provide temporal scanning without the need for any mechanical motion. These include:
Free-scanning lasers (A. Black, R. B. Apte, and D. M. Bloom,
Rev. Sci, Instrum.
63, 3191 (1992); K. S. Giboney, S. T. Allen, M. J. W. Redwell, and J. E. Bowers; “Picosecond Measurements by Free-Running Electro-Optic Sampling.”
IEEE Photon. Tech. Lett.,
6, pp. 1353-5, November 1994; J. D. Kafka, J. W. Pieterse, and M. L. Watts; “Two-color subpicosecond optical sampling technique.”
Opt. Lett.,
17, pp. 1286-9, Sep. 15, 1992 (hereinafter Kafka et al.); M. H. Ober, G. Sucha, and M. E. Fermann; “Controllable dual-wavelength operation of a femtosecond neodymium fiber laser,”
Opt. Lett.
20, pp. 195-7, Jan. 15, 1995).
Stepped mirror delay lines employing acousto-optic deflectors as the dispersive elements (R. Payaket, S. Hunter, J. E. Ford, S Esener; “Programmable ultrashort optical pulse delay using an acousto-optic deflector.” Appl.
Opt.,
34, no. 8, pp. 1445-1453, Mar. 10, 1995).
Slewing of RF phase between two modelocked lasers (D. E. Spence, W. E. Sleat, J. M. Evans, W. Sibbett, and J. D. Kafka; “Time synchronization measurements between two self-modelocked Ti:sapphire lasers.” Opt.
Comm.,
101, pp. 286-296, Aug. 15, 1993).
The non-mechanical methods, in particular, are capable of high speed scanning. The free-scanning lasers produce a scan range which spans the entire repetition period of the laser. For example, a known free-scanning laser system is shown in
FIG. 1
, which includes a master laser
10
and a slave laser
20
having different cavity lengths which produce pulse trains at different repetition frequencies v
1
and v
2
. The scan frequency is equal to the frequency difference &Dgr;v=v
1
−v
2
, and is set at the desired value by adjusting the cavity length of the slave laser to a specific fixed length. A correlator
40
produces a signal from the cross-correlation between the two lasers which gives information about the timing between the two lasers, and provides triggering signal to data acquisition electronics
50
. For example, in Kafka et al. two independent, mode-locked Ti:sapphire lasers, namely, master laser
10
and slave laser
20
each with a nominal repetition rate of 80 MHz, were set to have different repetition frequencies (by about 80 Hz). Due to the offset in repetition frequency, the lasers scanned through each other at an offset frequency &Dgr;v of approximately 100 kHz. This offset frequency can be stabilized to a local RF oscillator. Since the laser repetition rates were near 80 MHz, the total scan range was about 13 nsec. Thus, time scanning was achieved without employing any moving mechanical delay lines. Timing calibration was achieved by cross-correl

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