Coherent light generators – Particular resonant cavity – Specified cavity component
Reexamination Certificate
1999-07-21
2001-07-10
Davie, James W. (Department: 2881)
Coherent light generators
Particular resonant cavity
Specified cavity component
C372S006000, C372S018000
Reexamination Certificate
active
06259719
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to semiconductive saturable absorbers, and more particularly, to such absorbers for use as Bragg reflector mirrors in mode-locked lasers, and to the resulting mode-locked lasers.
BACKGROUND OF THE INVENTION
Mode-locked lasers are particularly useful for the generation of ultrashort optical pulses, for example pulses of widths in the picosecond and subpicosecond range, and typically of high energy. Semiconductive saturable absorbers are advantageously employed in such mode-locked lasers as Bragg-reflector mirrors.
A saturable absorber absorbs all weak incident radiation but when the intensity of the radiation is built up to a sufficiently high level, termed the saturation intensity, the saturable absorber is bleached and becomes transparent, and then incident radiation is permitted passage with relatively low attenuation. A saturable Bragg absorber is a non-linear element that acts on incident radiation as a shutter whose opacity changes as a function of the intensity of the incident radiation at a particular wavelength. When used in a laser as a Bragg-reflector mirror, the saturable Bragg absorber acts in its opaque state as a low-loss reflector of the incident stimulated emission of the laser, and so can serve as one end wall of its resonant cavity.
U.S. Pat. No. 5,627,854 that issued to applicant W.H. Knox on May 6, 1997 describes a mode-locked laser in which the mode-locking element is a saturable Bragg reflector incorporating a quantum well within the one or more low-index layers the top of a stack advantageously of at least twenty of quarter optical path wave-length layers alternately of low and high index of refraction. The resulting non-linear reflector provides an intensity-dependent response that permits it to be used for saturable absorption directly in the main oscillating cavity of the laser.
In many applications the gain medium of the laser is quite small allowing the absorber material, the quantum wells, to be few in number, and in such applications the absorber material little influences the optical design of the dielectric Bragg-reflector mirror in which the absorber material is placed. However for a high gain laser where a large modulation depth is desired for the saturable Bragg reflector, the prior art saturable Bragg reflector design generally requires expanding the number of absorber layers that need to be incorporated, often to ten or more. In this case the absorber layer section becomes a significant factor in the mirror design and tends to introduce significant optical scattering because of the dielectric discontinuities introduced by the absorber layers. In fact when many quantum wells are placed inside the dielectric quarter optical path wavelength layers, there must be increased the total number of quarter wavelength layers and this leads to larger scattering penalties and non-optimized positioning of the quantum wells within the stack.
The present invention seeks a more efficient way of introducing the quantum well absorber layers into the saturable Bragg reflectors for use in mode-locked lasers.
SUMMARY OF THE INVENTION
We have discovered an improved form of saturable Bragg reflector for use in mode-locked lasers. In particular, in our new reflector, the top of the stack is modified by making preferably at least the two topmost high index layers of an optical thickness of one eighth of the operating (optical path) wavelength and locating a quantum well in the high index layer of optical thickness of one quarter the operating wavelength that is sandwiched between low index layers of one eighth the operating wavelength. It is found that the insertion of the quantum well, because of its relatively narrow width, little affects the effective optical thickness of the layer in which it is inserted so that in practice its presence in the layer can be discounted in preparing the thickness of such layer. Moreover this technique is scalable by the addition of similar one quarter wavelength layers with quantum wells sandwiched between one-eighth wavelength layers to achieve still deeper modulation depths for obtaining especially short pulse widths.
The invention will be better understood from the following more detailed description taken in conjunction with the accompanying drawing.
REFERENCES:
patent: 5627854 (1997-05-01), Knox
patent: 5701327 (1997-12-01), Cunningham et al.
patent: 5901162 (1999-05-01), Alcock et al.
patent: 5987049 (1999-11-01), Weingarten et al.
patent: 6072811 (2000-06-01), Fermann et al.
Cunningham John E.
Knox Wayne H.
Davie James W.
Lucent Technologies
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