Coherent light generators – Particular pumping means – Pumping with optical or radiant energy
Reexamination Certificate
2000-02-11
2004-05-11
Ip, Paul (Department: 2828)
Coherent light generators
Particular pumping means
Pumping with optical or radiant energy
C372S099000, C372S018000, C372S050121, C372S049010, C372S010000, C372S013000, C372S036000, C372S074000
Reexamination Certificate
active
06735234
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to pulsed lasers and to methods for generating pulsed laser radiation and, more particularly, to passively. mode-locked optically pumped semiconductor external-cavity surface-emitting lasers (OPS-EXSELs).
BACKGROUND OF THE INVENTION
Semiconductor lasers are known in the art. Their laser gain medium consists of a semiconductor material such as InGaAs. In most cases, they do not require any external resonator because the end faces of the semiconductor material can be designed as the resonator mirrors. They can be pumped electrically by applying an appropriate voltage to the semiconductor material. The so-called bandgap engineering, a technique making use of the large number of known semiconductor materials and laser designs, offers a great variety of emittable wavelengths in the infrared and visible range. Semiconductor lasers are small and compact, and can be manufactured in great masses at low costs.
Semiconductor lasers can be designed either as edge-emitting lasers or as surface-emitting lasers. Edge-emitting lasers are the most common form of semiconductor lasers, but this concept very much limits the mode area in the device. For ultrashort pulse generation, a consequence of this is that the pulse energy is also limited to values far below what is achievable, e.g., with lasers based on ion-doped crystals. Also a high average output power (a few watts or more) cannot be generated with good transverse beam quality. These problems can be solved with surface-emitting semiconductor lasers, where the mode areas can be greatly increased, particularly if the device is optically pumped.
Electrically pumped vertical-cavity surface-emitting lasers (VCSELs) known to date are limited in their output power or in terms of beam quality. That is because in a small-area VCSEL the heat dissipation limits the driving current, while in a large-area VCSEL the pump distribution is not uniform enough to support fundamental-transversal-mode operation. With optical pumping the problem of the pump uniformity can be overcome and an external cavity ensures stable fundamental mode operation even with a large mode size. (M. Kuznetsov et al., “High-power (>0.5-W CW) diode-pumped vertical-external-cavity surface-emitting semiconductor lasers with circular TEM
00
beams”, IEEE Phot. Tech. Lett.,Vol. 9, No. 8, p. 1063, 1997) The extensive gain bandwidth of semiconductor quantum well lasers is attractive for ultrashort pulse generation. Lasers emitting short (in the nanosecond and sub-nanosecond range) or ultrashort (in the sub-picosecond range) pulses are known in the art. A well-known technique for short or ultrashort pulse generation is mode locking. Mode locking is a coherent superposition of longitudinal laser-cavity modes. It is forced by a temporal loss modulation which reduces the intracavity losses for a pulse within each cavity-roundtrip time. This results in an open net gain window, in which pulses only experience gain if they pass the modulator at a given time. The loss modulation can be formed either actively or passively. Active mode locking is achieved, for instance, using an acousto-optic modulator as an intracavity element, which is synchronized to the cavity-roundtrip time. Active mode locking of a diode-pumped quantum well laser has, e.g., been achieved with an intra-cavity acousto-optic prism, giving pulse lengths of 100-120 ps (M. A. Holm, P: Cusumano, D. Burns, A. I. Ferguson and M. D. Dawson, CLEO '99 Technical Digest, Baltimore 1999, paper CTuK63).
However, ultra-short-pulse generation relies on passive mode-locking techniques, because only a passive shutter is fast enough to shape and stabilize ultrashort pulses. Passive mode locking relies on a saturable absorber mechanism, which. produces decreasing loss with increasing optical intensity. When the saturable-absorber parameters are correctly adjusted for the laser system, stable and self-starting mode locking is obtained. Saturable-absorber mode locking of diode lasers has been widely investigated, originally using a semiconductor saturable absorber mirror (SESAM) in an external cavity (Y. Silberberg, P. W. Smith, D. J. Eilenberger, D. A. B. Miller, A. C. Gossard and W. Woiegan, Opt. Lett. 9, 507, 1984), and more recently in monolithic devices, which use sections of reverse-biased junction to provide saturable absorption (for a review, see “Ultrafast Diode Lasers: Fundamentals and Applications”, edited by P. Vasilev, Artech House, Boston, 1995). A harmonically mode-locked monolithic laser was shown to generate picosecond pulses at a repetition rate variable up to 1.54 THz (S. Arahira, Y. Matsui and. Y. Ogawa, IEEE J. Quantum Electron. 32, 1211, 1996); however such devices are limited to a few tens of milliwatts of output power.
Another approach for short-pulse generation was to use a mode-locked dye or solid-state laser as a synchronous optical pump for a vertical-external-cavity surface-emitting laser (VECSEL) (W. B. Jiang, R. Mirin and J. E. Bowers, Appl. Phys. Lett. 60, 677, 1992). These lasers typically produced chirped pulses with a length of about 20 ps, which were externally compressed to sub-picosecond, and even sub-100-femtosecond duration (W. H. Xiang, S. R. Friberg, K. Watanabe, S. Machida, Y. Sakai, H. Iwamura and Y. Yamamoto, Appl. Phys. Lett. 59, 2076, 1991). The general drawback of this approach, which prevents widespread applications, is that the pumping laser itself has to deliver ultrashort pulses. This severely limits the attractiveness of the overall system in terms of complexity, size, cost, and achievable pulse repetition rate.
In U.S. Pat. No. 5,461,637 (Mooradian et al.), a vertical-cavity surface-emitting laser (VCSEL) is disclosed with a quantum-well region formed over a semiconductor substrate. A first reflective surface is formed over the quantum-well region, and a second reflective surface is formed over the substrate, opposite the first reflective surface, forming a laser cavity. However, there is no teaching about measures to be taken for mode locking such a VCSEL.
SUMMARY OF THE INVENTION
It is an object of this invention to provide a simple, robust laser emitting short (in the picosecond range) or ultrashort (in the sub-picosecond range) pulses, with a high repetition rate (in the range of a few GHz or higher), with a high optical average output power (of at least hundreds of milliwatts) and a good beam quality (coefficient of beam quality M
2
≦5; cf. T. F. Johnston, Jr., “M
2
concept characterizes beam quality”, Laser Focus World, May 1990).
It has been found that the combination of an optically pumped external-cavity surface-emitting laser (EXSEL) with a semiconductor saturable absorber structure solves the above problem. Thus the laser according to the invention comprises a surface-emitting semiconductor laser with an external cavity. The laser is pumped optically, preferably with a high-power diode laser bar. Finally, it is passively mode-locked with a SESAM in the external cavity, or alternatively with a saturable absorber which is incorporated into the semiconductor laser structure. SESAM stands here for any semiconductor saturable absorber structures, which have sometimes been termed A-FPSA (Opt. Lett. 17, 505, 1992), SBR (Opt. Lett. 20, 1406, 1995), D-SAM (Opt. Lett. 21, 486, 1996), semiconductor doped dielectric layers (Opt. Lett. 23, 1766, 1998), or colored glass filters (Appl. Phys. Lett. 57, 229, (1990), for example. Any other saturable absorbers could be used which allow to adjust the operation parameters for stable mode locking (cf. C. Hönninger et al., “Q-switching stability limits of cw passive mode locking”, J. Opt. Soc. Am. B 16, 46, 1999).
More particularly, the laser according to the invention comprises:
a first reflective element and a second reflective element being separated therefrom, said first and second reflective elements defining an optical resonator for laser radiation;
an essentially plane semiconductor gain structure having a surface extending essentially in a surface plane, for emitting said laser radiation;
me
Haering Reto
Keller Ursula
Paschotta Ruediger
Flores Ruiz Delma R
Giga Tera AG
Ip Paul
Oppedahl & Larson LLP
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