Semiconductor saturable absorber device, and laser

Details Semiconductor saturable absorber device, and laser Semiconductor saturable absorber device, and laser

2002-03-14

2004-11-30

Ip, Paul (Department: 2828)

Coherent light generators

Particular active media

Semiconductor

C372S045013

Reexamination Certificate

active

06826219

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 pulses—i.e. pulses in the picosecond and in the sub-picosecond range—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, for ultra-short-pulse generation, passive mode-locking techniques are preferred, because only a passive shutter is fast enough to shape and stabilize ultrashort pulses. One option to implement passive mode locking is to rely 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.
A broad class of semiconductor saturable absorbers are known in the state of the art. Such saturable absorbers usually comprise a layered structure with one or several layers of a semiconductor material having a non-linear absorption characteristics at the laser frequency. By choosing appropriate layers with specifically prepared surfaces, a large variety of different optical properties can be achieved for such structures. Especially, these structures may be designed to be anti-resonant or resonant, and they may have a high Q-factor or a low Q-factor. For mode-locking in lasers, antiresonant devices have been the structures of choice. This is because resonant structures have much narrower tolerances of growth accuracy and the high field intensities in the structures lead to high losses and a delicate dependence of the characteristics of the entire laser system on the absorber device properties such as its quality etc. of the structure. Known structures of this kind include the antiresonant Fabry-Perot Saturable Absorbers (A-FPSAs), and conventional semiconductor saturable absorber devices (cf. U. Keller et al., “Semiconductor saturable absorber mirrors 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) being low Q antiresonant devices. Further examples of structures include the Saturable Bragg Reflector (SBR). Recently, a low-field enhancement (i.e. low Q) but resonant saturable absorber device (LOFERS) design has been invented. A corresponding U.S. patent application by Weingarten, Spühler, Keller, and Krainer is pending and has been attributed application Ser. No. 10/016,530.
Jung et. Al. have disclosed a “thin-absorber” device for operating around 840 nm which uses a layer of dielectric to complete the semiconductor structure. This layer is described as an effective “anti-reflection” (AR) coating. (Electronics Letters Feb. 1995, pp. 288-289).
For many applications, InGaAs is a preferred absorber material due to its inherent properties. This, however, brings about new challenges when applied to lasers designed for communication technology purposes. In communication technology, lasers operating at frequencies corresponding to a free space wavelength of 1.55 &mgr;m are increasingly important. One of the challenges with 1.55 micron operation of semiconductor saturable absorber devices is the high concentration of In in the InGaAs absorber layer, required to achieve absorption at this wavelength. InGaAs is the material of choice in many known saturable absorbers. However, in order to cause the absorption edge to be energetically as low as 1.55 &mgr;m, the concentration of the In replacing the Ga when going from GaAs to InGaAs has to be rather high, i.e. the absorber material is In
x
Ga
1−x
As with x~50%-58%, and GaAs or AlAs. The admixture of In in such a high concentration, of course, also changes other material properties than the bandgap, one of them being the lattice constant. As a consequence, a much higher lattice mismatch has to be dealt with in 1550 nm semi conductor saturable absorber devices than, for example, in 1060 nm semiconductor saturable absorber devices. For example, the natural, relaxed lattice constant of In
0.53
Ga
0.47
As is about 0.583 nm vs. 0.565 nm for GaAs and 0.566 nm for AlAs. InGaAs absorber layers grown onto or in GaAs or AlAs layers thus tend to relax—i.e. to re-adopt the natural InGaAs lattice constant at the price of a certain, high amount of generated defects—if a certain critical layer thickness is exceeded. The defects substantially reduce the device quality in terms of losses. Due to this lattice mismatch, most In
x
Ga
1−x
As (x>0.5) absorber layers will relax within 1-2 nm thickness, resulting in many defects. This brings about a decrease in the optical quality of the crystalline layers grown following the absorber layer, since defects tend to propagate through layers grown by epitaxy subsequently to the relaxed absorber layers.
A further important frequency used for telecommunication purposes corresponds to the free space wavelength of essentially 1.3 &mgr;m (i.e. the frequency equals the speed of light divided by about 1.3 &mgr;m.). In this case, the In concentration in the absorber material is lower, i.e. x~0.4. Although the lattice mismatch between In
x
Ga
1−x
As (x~0.4) and GaAs or AlAs is not as high as for 1.55 &mgr;m, ensuring epitaxial growth is still an important issue also in this system.
One method to avoid this is to grow lattice-matched layers. However, this requires using Bragg reflector materials such as InP/InGaAsP, or InP/AlGaInAs or AlInAs/AlGaInAs. The disadvantage of these material systems include a reduced index contrast between the mirror pairs, resulting in less reflectivity and less mirror reflectivity bandwidth for a given number of layer pairs (compared to GaAs/AlAs for example), more complex epitaxial growth processes in MOCVD or MBE machines, and increased losses in the structures, increased demands on the growth accuracy.
In U.S. Pat. No. 5,701,327, a semiconductor saturable absorber device for lasers operating at 1.55 micron is disclosed, which comprises a GaAs/AlAs Bragg mirror, onto which a InP “strain relief” layer is grown. The absorber layers are embedded in this InP strain relief layer. With InP grown on GaAs or AlAs, many defects are formed which may serve as recombination centers leading to an ultra-fast device response. However, such a semiconductor saturable absorber devices brings about comparably high losses.
A further challenge to be met with mode-locked solid-state lasers are Q-switching instabilities which are present at high frequencies. For passively mode-locked lasers using semiconductor saturable absorber mirror devices or similar devices for mode-locking, the onset of Q-switching instabilities limits the repetition rate (see U. Keller et al., “Semiconductor saturable absorber mirrors for femtosecond to nanosecond pulse generation in solid-state lasers,” Journal of Selected Topics in Quantum Electronics (JSTQE), vol. 2, no. 3, pp. 435-453, 1996; and U. Keller, “Ultrafast all-solid-state laser technology”,
Applied Physics. B,
vol. 58, pp. 347-363, 1994).
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,

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