Active solid-state devices (e.g. – transistors – solid-state diode – Responsive to non-electrical signal – Electromagnetic or particle radiation
Reexamination Certificate
2001-12-10
2003-03-25
Nelms, David (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Responsive to non-electrical signal
Electromagnetic or particle radiation
C257S432000, C257S431000, C257S464000, C438S048000, C438S065000, C438S054000, C438S069000, C372S018000
Reexamination Certificate
active
06538298
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a semiconductor device and, more particularly, to a semiconductor saturable absorber device for use in mode-locked lasers for the generation of short and ultrashort optical pulses. The invention also relates to a mode-locked laser comprising a semiconductor saturable absorber device.
BACKGROUND OF THE INVENTION
Lasers emitting short or ultrashort (in the picosecond or 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. 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.
Passive mode locking can be achieved with semiconductor saturable absorber mirrors (SESAMs) (cf. U. Keller et al., “Semiconductor saturable absorber mirrors (SESAMs) for femtosecond to nanosecond pulse generation in solid-state lasers”, Journal of Selected Topics in Quantum Electronics (JSTQE), Vol. 2, No. 3, 435-453, 1996, incorporated herein by reference). A SESAM is a nonlinear mirror inserted inside the laser cavity. Its reflectivity is higher at higher light intensities due to absorption bleaching obtained by using semiconductors as the nonlinear material. A SESAM typically comprises a bottom mirror and the saturable absorber structure. Optionally, there may be a spacer layer and/or an additional antireflection or reflecting coating on the top surface.
For passively mode-locked lasers using SESAMs for mode-locking, the limitation on repetition rate is the onset of Q-switching instabilities (see C. Hönninger et al., “Q-switching stability limits of continuous-wave passive mode locking,” J. Opt. Soc. Am. B. vol. 16, pp. 46-56, 1999). This has also limited the laser repetition rate to the range of several hundred megahertz typically. Using the technique of coupled cavity mode-locking (RPM), a repetition rate of 1 GHz was demonstrated (see U. Keller, “Diode-pumped, high repetition rate, resonant passive mode-locked Nd:YLF laser”, Proceedings on Advanced Solid-State Lasers, vol. 13, pp. 94-97, 1992). However this is a much more complicated laser due to the additional laser cavity which has to be carefully aligned with the main laser cavity.
When the conditions necessary to avoid the Q-switching instabilities in passively mode-locked lasers are examined more carefully, the following stability condition can be derived:
(
F
laser
/F
sat,laser
).(
F
abs
/F
sat,abs
)>&Dgr;
R
(1)
where F
laser
is the fluence in the laser material, F
sat,laser
=h&ugr;/&sgr;
laser
is the saturation fluence of the laser material, h is Planck's constant, &ugr; is the center laser frequency, &sgr;
laser
is the laser cross-section parameter (see W. Koechner,
Solid-State Laser Engineering
, 4
th
Edition, Springer-Verlag New York, 1996), F
abs
is the fluence on the absorber device, F
sat,abs
=h&ugr;/&sgr;
abs-eff
is the effective saturation fluence of the absorber, where &sgr;
abs-eff
is the effective cross-section parameter of the absorber device including a structure dependent factor and the intrinsic material cross section, and &Dgr;R is the modulation depth of the absorber device. This equation can be used to scale a laser for operation at higher repetition rates. If all else remains constant (i.e., mode size in laser material and on the absorber, average power, and pulsewidth), as the repetition rate increases, the left-hand term decreases due to decreasing pulse energy. It is possible to avoid Q-switching under this condition by arbitrarily decreasing the modulation depth &Dgr;R. However, below a certain modulation depth, the absorber will not have a strong enough effect to start and sustain mode-locking.
For further clarity we simplify Eq. (1) to the following:
S
laser
. S
abs
>&Dgr;R
(2)
where S
laser
is the fluence ratio in the laser material, and S
abs
is the fluence ratio on the absorber. This reduced notation allows us to simplify the further discussion. To achieve the maximum figure of merit, one can change the laser design to increase the fluence ratio S
laser
in the laser material, or to increase the fluence ratio S
abs
in the absorber. In this document, we concentrate on the latter measure.
In pulse generating lasers with high repetition rates, e.g. above 1 GHz, the pulse energy of course is lower for a given average power from the laser. Thus, as the pulse repetition rate goes up, it becomes increasingly harder to saturate the SESAM and thus to get modelocking. It would therefore be desirable to obtain an absorber device with a decreased saturation fluence for high repetition rate pulse generating lasers.
A reduced saturation fluence would make operation with a reduced fluence level on the SESAM possible. The beam spot size on the absorber medium could be chosen to be larger. This would be desirable for both pulse generating lasers with a high repetition rate and for pulse generating lasers operating at a high average power: A larger spot size on the absorber make cavity design easier and, very importantly, be an advantage concerning thermal issues. A very high fluence can result in optical damage. Damage levels of SESAM absorbers have been measured in the range of 30 mJ/cm
2
. By decreasing the saturation fluence of the absorber and by then increasing the spot size on the absorber, the laser can be kept well off the damage level. Next to possible thermal damages, a very high fluence (but still below the damage threshold) may cause the laser to operate with multiple pulses per round trip, i.e., a form of harmonic mode-locking. This may be desirable as a method to increase the repetition rate of the laser. However, it may result in decreased operation stability of the laser. Thus, S
abs
is limited to about 10-30 for fundamental mode locking.
Minimum saturation fluence can be achieved by positioning the absorber medium at or near the peak of the standing wave in the SESAM.
Other absorber materials with higher cross sections, i.e., lower saturation fluences, could be found in theory. However, this is a very difficult material problem, the solution of which in the near of even far future is uncertain.
In the paper “Erbium-Ytterbium Waveguide Laser Mode-Locked with a Semiconductor Saturable Absorber Mirror”, IEEE Photonics Technology Letters, Vol. 12, No. 2, February 2000. E. R. Thoen et al. propose the use of a SESAM for mode-locking a waveguide laser with a resonant, multi-layer dielectric coating in order to increase the absorption and to lower the saturation fluence. A resonant structure as proposed in this paper, however, brings about high losses. It is therefore only useful in set-ups with very high gain such as the waveguide lasers disclosed in this paper. In addition, the resonance condition makes it delicate to fabricate.
More generally, resonant Fabry-Perot saturable absorbers are considered to be unsuitable as modulators for mode-locked lasers, because the loss and the group delay dispersion (GDD) which they introduce are quite high (cf. M. J. Lederer et al., “An Antiresonant Fabry-Perot Saturable Absorber for Passive Mode-Locking Fabricated by Metal-Organic Vapor Phase Epitaxy and Ion Implantation Design, Characteriza
Keller Ursula
Krainer Lukas
Spuehler Gabriel J.
Weingarten Kurt
Gigatera AG
Huynh Andy
Rankin, Hill Porter & Clark LLP
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