Coherent light generators – Particular resonant cavity – Specified cavity component
Reexamination Certificate
2000-08-01
2003-07-08
Scott, Jr., Leon (Department: 2828)
Coherent light generators
Particular resonant cavity
Specified cavity component
C359S588000, C359S584000, C372S029023
Reexamination Certificate
active
06590925
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to dielectric mirrors used in laser systems.
An objective of certain types of laser systems is generation of extremely short laser pulses, such as femtosecond pulses. Femtosecond laser pulses are useful in a wide range of technologies, including signal processing, high speed communications, optical imaging, and optical accelerators. As laser system engineers continue to generate shorter and shorter pulses, the frontiers of the above technologies continue to expand.
Laser systems that generate broadband ultra-short pulses must include reflective structures, e.g., mirrors, that achieve high reflectivity over a broad wavelength range. In general, the broadband reflective mirrors in such laser systems are Bragg mirrors that have been modified to control group delay dispersions (GDDs).
Referring to
FIG. 1
, a standard dielectric Bragg mirror
10
includes alternating high refractive index and low refractive index layers, such as alternating TiO
2
/SiO
2
layers
12
and
14
. Each layer has a thickness of &lgr;
B
/4, where &lgr;
B
is the Bragg wavelength of the mirror. The high reflectivity bandwidth of standard Bragg mirror
10
, however, is only about 200 nm at a center wavelength of 800 nm. The useful high reflectivity bandwidth is further limited (e.g., to about 100 nm) by higher order GDDs produced by standard Bragg mirrors. The bandwidth of an ultra-short pulse generated by a laser system using mirror
10
, therefore, will also be unacceptably limited.
To expand the useful high reflectivity bandwidth of standard Bragg mirrors, designers began “chirping” the layer pairs in the mirror. Referring to
FIG. 2
, in a simple chirped mirror
30
, the thickness T
n
of individual layer pairs varies along the length of the mirror, shortening toward the front
32
of mirror
30
. As a result, longer wavelengths penetrate deeper into the mirror than shorter wavelengths before being reflected, allowing mirror
30
to reflect an enlarged wavelength range. In addition, the reflection includes a negative dispersion, since the longer wavelengths experience more group delay than the shorter wavelengths. This dispersion compensates for the positive dispersion produced by other cavity components in the laser system, such as the laser crystal.
It turns out, however, that simple chirped mirrors do not produce a smooth, controlled group delay. While the local average of the group delay does increase linearly with increasing wavelength, as expected, it also exhibits strong oscillations. The cause of these oscillations is the following. Longer wavelengths (e.g., &lgr;
2
in
FIG. 2
) have to pass the first section of the Bragg mirror, which acts as a transmission grating for these wavelengths. Slight reflections of &lgr;
2
from the front section of mirror
30
, therefore, interfere with stronger reflections of &lgr;2 from the back layers, as in a Gires-Tournouis Interferometer (GTI). The oscillations in the group delay caused by the GTI effect have an amplitude of several tens of femtoseconds, making these simple-chirped mirrors less useful for ultra short pulse generation. See Kartner et al., WO 99/60675, which is incorporated herein by reference, and Matuschek et al., “Theory of Double-Chirped Mirrors,”
IEEE J. of Selected Topics in Quantum Electronics
, 4:197-208 (1998).
To compensate for the GTI effect experienced by the longer wavelengths, engineers developed double-chirped mirrors. Referring to
FIG. 3
, a double-chirped mirror
50
has about 60 alternating high and low refractive index layers
52
and
54
. (For clarity,
FIG. 3
shows only 24 layers.) As in the simple chirped mirror, the thickness of individual layer pairs varies along the length of mirror
50
, decreasing towards a front
56
of the mirror. In addition, the thickness of the high index layers
52
varies relative to the low index layers
54
, such that the difference in thickness between the layers in each pair increases towards front
56
. This gradual variation in the relative thickness of the high index layers
52
causes a gradual increase, or “chirping,” in the coupling coefficient in the front portion, or “double-chirped” portion
58
, of mirror
50
. If the coupling coefficient is chirped along with the period of the grating, then the GTI effect caused by the impedance mismatch in portion
58
of the mirror can be effectively eliminated, thereby substantially reducing oscillations in the group delay found in simple chirped mirrors. Double-chirped mirrors are further described in Matuschek et al., “Analytical Design of Double-Chirped Mirrors with Custom-Tailored Dispersion Characteristics,”
IEEE J. of Quantum Electronics
, 35:129-37 (1999); and Matuschek et al. (1998), supra, both of which are incorporated herein by reference.
While double chirping does substantially reduce oscillations caused by the impedance mismatch within the double-chirped portion
58
of mirror
50
, it does not produce an entirely controlled group delay. The reason is that a second impedance mismatch exists in mirror
50
, between the air and front
56
of the mirror. The refractive index jump between air and the first layer
60
at front
56
introduces a reflection and, consequently, a second GTI-like oscillation in the group delay. Matuschek et al. (1998), supra.
To reduce the oscillations caused by the air-mirror mismatch, engineers add a multi-layer anti-reflective (AR) coating
62
to front
56
of the mirror. Id. While the AR coating does taper the impedance, it does not entirely alleviate the mismatch. For a typical laser system, the AR coating
62
must have a very low amplitude reflectivity, r, e.g., less than 0.01, or preferably less than 0.001, to effectively reduce the oscillations caused by the air-mirror mismatch. At present, AR coatings with amplitude reflectivities less than 0.01 are expensive, and can only be achieved over a wavelength range of about 350 nm. AR coatings with amplitude reflectivities below 0.001 are not yet possible. Double-chirped mirrors with AR coatings, therefore, do not adequately reduce GTI-like oscillations caused by the air-mirror mismatch over a bandwidth greater than about 350 nm at a center wavelength of 800 nm, which is about half an octave in the frequency domain.
SUMMARY OF THE INVENTION
In general, in one aspect, the invention features a mirror system for use in generating a short duration laser pulse. The system includes first and second double-chirped mirrors disposed along an optical path within a cavity, where the second double-chirped mirror includes an additional phase-shifting layer as compared to the first double-chirped mirror. The additional phase-shifting layer causes the mirror system during use to produce a laser pulse that is characterized by oscillations in group delay substantially reduced in amplitude in comparison to oscillations in group delay for a pulse produced by the same system without the additional phase-shifting layer.
Embodiments of this aspect of the invention may include one or more of the following features. The additional phase-shifting layer can have a thickness of about ¼ of a center wavelength of the mirror. The mirrors can be computer optimized so that reflections from the mirrors have equal average group delay dispersions, but opposite oscillations in group delay dispersion over substantially all wavelengths reflected by the mirrors.
The oscillations in group delay substantially reduced in amplitude by inclusion of the additional phase-shifting layer are caused by, e.g., impedance mismatches between air and the double-chirped mirrors. The additional phase-shifting layer reduces overall oscillations by causing the oscillations in group delay resulting from the impedance mismatch between air and the second double-chirped mirror to be out of phase (e.g., by &pgr; over all wavelengths reflected by the mirrors) with oscillations in group delay resulting from the impedance mismatch between air and the first double-chirped mirror.
In another aspect, the invention features a mirror system for use in generatin
Fujimoto James G.
Ippen Erich P.
Kaertner Franz X.
Morgner Uwe
Fish & Richardson P.C.
Jr. Leon Scott
Massachusetts Institute of Technology
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