Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
1999-07-13
2001-07-03
Lee, John D. (Department: 2874)
Optical waveguides
With optical coupler
Input/output coupler
C385S015000, C385S031000, C359S584000
Reexamination Certificate
active
06256434
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the field of reflective dielectric and/or semiconductor devices, and more particularly to broadband reflective dielectric and/or semiconductor devices used as mirrors in laser resonators.
BACKGROUND OF THE INVENTION
Chirped minors have been established in various set-ups for the generation of ultrashort laser pulses (see, e.g., U.S. Pat. No. 5,734,503 (Szipöcs et al.); R. Szipöcs, K. Ferencz, Ch. Spielmann, and F. Krausz,
Opt. Lett.
19, 201, (1994); G. Tempea, F. Krausz, Ch. Spielmann, and K. Ferencz,
IEEE JSTQE
4, 193, (1997); F. X. Kärtner, N. Matuschek, T. Schibli, U. Keller, H. Haus, C. Heine, R. Morf, V. Scheuer, M. Tilsch and T. Tschudi,
Opt. Lett.
22, 831 (1997)). Their high-reflectance bandwidth is broader compared to standard dielectric quarter-wave Bragg mirrors and their dispersion can be custom-tailored such that the dispersion of other elements in the laser is compensated for. Thus, chirped mirrors are very useful and compact devices for broadband dispersion control in systems like modelocked laser oscillators, laser amplifiers, parametric oscillators and amplifiers, and dispersive delay lines.
The basic idea behind a chirped mirror is relatively simple: layers with increasing thicknesses (e.g., quarter-wave layers with a gradually increasing Bragg wavelength) are stacked such that longer wavelengths penetrate deeper into the mirror structure, producing a negative group delay dispersion (GDD). The whole principle can also be inverted by decreasing the layer thicknesses, which results in chirped mirrors with a positive GDD.
Besides all advantages, chirped mirrors have one major disadvantage, as far as has been demonstrated. The designed group delay (GD) and GDD generally show unwanted oscillations around their desired target functions. One finds that, unfortunately, the amplitude of the oscillations dramatically increases with an increasing bandwidth. Typically, for standard chirped mirrors, these oscillations are reduced as much as possible by more or less sophisticated numerical optimization techniques (R. Szipöcs, K. Ferencz, Ch. Spielmann, and F. Krausz,
Opt. Lett.
19, 201, (1994); G. Tempea, F. Krausz, Ch. Spielmann, and K. Ferencz,
IEEE JSTQE
4, 193, (1997); R. Szipöcs, and A. Kohazi-Kis,
Apl. Phys.
B 65, 115 (1997)). Up to a certain limit, this approach has been successfully applied on chirped mirrors used in various types of lasers (see, e.g., L. Xu, Ch. Spielmann, F. Krausz, and R. Szipöcs,
Opt. Lett.
21, 1259 (1996); E. J. Mayer, J. Möbius, A. Euteneuer, W. W. Rühle, and R. Szipöcs,
Opt. Lett.
22, 528 (1997); A. Baltuska, Z. Wei, M. S. Pshenichnikov, D. A. Wiersma, and R. Szipöcs,
Appl. Phys.
B 65, 175 (1997); M. Nisoli, S. De Silvestri, O. Svelto, R. Szipöcs, K. Férenz, Ch. Spielmann, S. Sartania, and F. Krausz,
Opt. Lett.
22, 522 (1997)). However, designing by computer optimization leads to no deeper insight into the physics of a chirped mirror and the related design problems. In particular, the physical origin for the resulting oscillations remains unclear. Additionally, mirror structures obtained by pure computer optimization might be plagued by internal resonances (R. Szipöcs and A. Kohazi-Kis,
Apl. Phys.
B 65, 115 (1997)).
Recently, a theory of chirped mirrors was developed which explains the oscillations as caused by an impedance mismatch in the front part of the mirror (F. X. Kärtner, N. Matuschek, T. Schibli, U. Keller, H. Haus, C. Heine, R. Morf, V. Scheuer, M. Tilsch and T. Tschudi,
Opt. Lett.
22, 831 (1997); N. Matuschek, F. X. Kärtner, and U. Keller,
Conference on Laser and Electrooptics
(CLEO '98), San Francisco, Calif., May 3-8, paper CThC6 (1998); N. Matuschek, F. X. Kärtner, and U. Keller,
IEEE J. Sel. Topics Quantum Electron,
4, 197 (1998); N. Matuschek, F. X. Kärtner, and U. Keller,
IEEE J. Quantum Electron.
35, 129 (1999)). This mismatch of the characteristic impedance is responsible for the formation of an effective Gires-Tournois-interferometer (GTI), i.e., slight reflections in the front section interfere with strong reflections from the back, leading to oscillations well known for a GTI. In principle, for the design of a chirped mirror, the mismatch problem can be decomposed into two different matching problems effectively giving rise to two separate GTIs:
a) A GTI is formed due to an impedance mismatch of the designed stricture to the theoretically assumed refractive index of the ambient medium. For the generation of an initial design, it might be advantageous, to assume an ambient medium that is different from the actual ambient medium, which is typically air.
b) An additional GTI-like effect occurs due to the impedance mismatch between the theoretically assumed refractive index and the actual refractive index of the ambient medium.
We note that two special cases are included in both matching problems. If an initial design is obtained under the assumption that the ambient medium is air, the matching problem b) does not exist. In the same way, for an initial design that is obtained for an arbitrary refractive index, the matching problem b) also does not exist, if the refractive index of the ambient medium is identical to the theoretically assumed index for the ambient medium. In principle, the assumed refractive index can have any value that is appropriate for the generation of the theoretical design. From the theoretical point of view, for the case of a binary multilayer system, promising candidates are the refractive index of the low- or high-index layer, the arithmetic or geometric average refractive index, or the effective refractive index. However, the choice is not limited to these indices. A typical solution for problem b) is a multilayer subsection, which acts as an antireflection (AR) coating, that matches the theoretically assumed refractive index to the index of the ambient medium. We point out that for practical designs no absolutely perfect solution for either of the matching problems exists.
As mentioned above, in a standard chirped mirror one tries to solve both matching problems simply by using special optimization algorithms for the reduction of unwanted oscillations. It is obvious that this uncontrolled method of solving the involved matching problems does not lead to mirror structures with an optimum design performance. The theory derived in previous publications (F. X. Kärtner, N. Matuschek, T. Schibli, U. Keller, H. Haus, C. Heine, R. Morf, V. Scheuer, M. Tilsch and T. Tschudi,
Opt. Lett.
22, 831 (1997); N. Matuschek, F. X. Kärtner, and U. Keller,
Conference on Laser and Electrooptics
(CLEO '98), San Francisco, Calif., May 3-8, paper CThC6 (1998); N. Matuschek, F. X. Kärtner, and U. Keller,
IEEE J. Sel. Topics Quantum Electron.
4, 197 (1998); N. Matuschek, F. X. Kärtner, and U. Keller,
IEEE J. Quantum Electron.
35, 129 (1999)) offers an analytical way for solving the matching problem a). One possibility to avoid GTI-like oscillations caused by problem a) is to use the double-chirped mirror (DCM) design technique.
A double-chirped mirror (DCM) is a multilayer interference coating that can be considered as a composition of four sections.
FIG. 1
is a schematic drawing of a standard DCM
101
according to the prior art, composed of four multilayer sections
104
,
131
-
133
deposited on a substrate
102
. Each of the sections
104
,
131
-
133
serves a different task (see, e.g., N. Matuschek, F. X. Kärtner, and U. Keller,
IEEE J. Quantum Electron.
35, 129 (1999)). The first section
104
is a broadband AR coating, typically consisting of 10-14 layers (not shown separately). This AR coating
104
solves the matching problem b), as mentioned above. In the specific case discussed here, it matches the refractive index of a low-index material
103
.
1
to the refractive index of an ambient medium
105
, e.g., air. The other sections
131
-
133
represent the actual DCM structure
103
, as derived from theory. Typically, the mirror structure
103
consists of a plurality of alternately low-index layers
103
.
11
,
103
Gallmann Lukas
Keller Ursula
Matuschek Nicolai
Steinmeyer Guenter
Sutter Dirk H.
Doan Jennifer
Lee John D.
Oppedahl & Larson LLP
Time-Bandwidth Products AG
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