Description of Related Art Semiconductor saturable absorbers have found applicability as modelocking elements in solid state lasers for generating extremely short duration optical pulses. These pulses are commonly called ultrashort pulses because they exhibit pulse widths in the picosecond and sub-picosecond ranges. Ultrashort optical pulses are useful for high speed signal processing and data communications.

The saturable absorber allows passive modelocking of a laser when the absorber, which is a nonlinear element, is placed either within the lasing optical cavity or in an optical cavity, coupled and external to the lasing cavity. Saturable absorbers act as shutters to incident radiation because they can change their opacity as a function of the intensity of the incident radiation at a particular wavelength. A saturable absorber can absorb all weak incident radiation. As the intensity of incident radiation reaches a sufficiently high level known as the saturation intensity. incident radiation is permitted passage through the saturable absorber. In general, attenuation caused by the absorber is relatively low because the absorber is saturated into a transparent state at the desired wavelength.

Semiconductor saturable absorbers have been fabricated for narrowband and broadband response. Bulk semiconductor material and multiple quantum well heterostructures have been used for narrowband absorption applications

while specially graded bandgap multiple quantum well heterostructures have been developed for broadband applications. In the quantum well realizations of such absorber devices, the quantum well heterostructure has been grown on a semiconductor quarter-wave stack reflector.

A saturable Bragg reflector, as disclosed in U. S. Patent No. 5,627,854, incorporated herein by reference, has been used to mode lock lasers near 800 nm. The'854 Patent discloses a Ti: sapphire laser that generates ultrashort optical pulses having a pulse autocorrelation of approximately 90 fs. The pulsewidth is determined by dispersion and bandwidth properties of the saturatable Bragg reflector. In contrast, for narrower bandwidth lasers, such as Nd: YV04, only picosecond pulses can be generated and the pulsewidth is determined by the modulation depth of the saturable Bragg reflector. In order to obtain the shortest pulses from this laser, a higher modulation depth is required.

There is a need for a Bragg reflector which has greater modulation depth.

SUMMARY OF THE INVENTION Accordingly, an object of the invention is to provide an intensity dependent reflector.

Another object of the invention is to provide a Bragg mirror for use in mode locking lasers for the generation of ultrashort optical pulses.

Still another object of the invention is to provide a laser that produces high power and short pulses simultaneously.

A further object of the invention is to provide a laser with an end mirror that includes a quarter-wave stack with at least two quantum-well-structures formed in different layers of the quarter-wave stack.

These and other objects of the invention are achieved in a laser for generating an optical beam at a first wavelength. First and second end reflectors define a laser cavity. The second end reflector includes a quarter-wave stack of material layers. A gain medium is positioned in the laser cavity. A first quantum-well-structure is included in one of the material layers of the quarter-

wave stack. A second quantum-well-structure is included in a different material layer of the quarter-wave stack.

BRIEF DESCRIPTION OF THE FIGURES Figure 1 is a cross-sectional view of a saturable Bragg reflector with a quarter-wave stack of material layers, and a quantum-well-structure with multiple quantum wells included in the top layer of the quarter-wave stack.

Figure 2 is a cross-sectional view of a saturable Bragg reflector with a quarter-wave stack of material layers, and quantum-well-structures with multiple quantum wells included in the top and third layers of the quarter-wave stack.

Figure 3 is a cross-sectional view of a saturable Bragg reflector with a quarter-wave stack of material layers, and quantum-well-structures with multiple quantum wells included in the top, third and fifth layers of the quarter- wave stack.

Figure 4 is a schematic diagram of a diode pumped Vanadate laser including a saturable Bragg reflector, as in Figures 1,2 or 3, as an intracavity element for modelocking.

DETAILED DESCRIPTION In one embodiment of the present invention, a saturable Bragg reflector is provided that acts as a low loss saturable absorber. The difference between the reflectivity when the device is saturated and when it is not saturated is called the modulation depth. A typical value for the modulation depth is 1%. This value is sufficient to cause mode-locking in most solid state lasers. For laser gain media, including but not limited to Nd: YVO4 (Vanadate), which emits at 1064 nm and has a bandwidth on the order of 0.1 nm, mode-locked pulses of about 10 picoseconds can be generated. The pulse duration depends on the balance between gain narrowing, which tends to broaden the pulse duration and the modulation caused by the saturable absorber, which shortens it. Thus, the larger the modulation depth, the shorter the pulse duration.

While most cw lasers may require only a 1-2% output coupling, high gain laser media, such as Vanadate produce optimum output power with an output coupling of 10-20%. Generally, as the value of the output coupling is increased the power from the laser increases as does the pulsewidth. This is because the modulation depth of the absorber (1 %) becomes a smaller effect relative to the output coupling (10%).

One embodiment of the present invention provides a laser that produces high power and short pulses simultaneously. A saturable Bragg reflector is provided with higher modulation depth. This requires a larger number of quantum wells. As used herein, a"quantum-well-structure"contains one or more quantum wells. A quantum-well-structure containing several quantum wells can be placed in the top quarter wave layer of the Bragg mirror to increase the modulation depth. The total optical thickness of the top layer is one quarter of an optical wavelength to preserve the high reflectivity of the Bragg mirror.

Even when the entire top quarter wave layer is consumed with quantum wells the pulsewidth can be further shortened by adding additional quantum wells. It is most straightforward to add quantum wells to the third layer of the quarter wave stack, thus preserving the optical properties of the Bragg mirror.

The total optical thickness of the third layer is also one quarter of an optical wavelength to preserve the high reflectivity of the Bragg mirror. To reduce the pulsewidth even further, the first, third and fifth layers of the quarter wave stack can be consumed with quantum wells. Strain balancing techniques are disclosed in Mat. Res. Soc. Symp. Proc., Vol. 379, page 475 91995) and U. S.

Patent No. 5,701,327, both incorporated herein by reference.

Referring now to Figures 1-3, a Bragg reflector 10 includes a substrate 12 and a stack 14 of alternating high and low index layers of quarter wave optical thickness.

Bragg reflector 10 illustrated in Figure 1, includes a quarter wave stack 14 where the top layer has been replaced with a quantum-well-structure 16 with multiple quantum wells.

Referring now to Figure 2, the third layer of quarter wave stack 14 has also been replaced with a quantum-well-structure 18. In Figure 3, the fifth layer of quarter wave stack 14 has also been replaced with a quantum-well-structure 20. It will be appreciated that multiple quantum-well-structures 16,18 and 20 can be formed in any non-adjacent layers of quarter wave stack 14.

In one preferred embodiment for operation at 1064 nm, substrate 12 is made of GaAs, the high and low index layers of stack 14 are made of GaAs and AlAs and quantum-well-structures 16,18 and 20 are made of InGaAs and GaAsP. The quantum wells can have differing thicknesses or compositions.

Figure 4 illustrates Bragg reflector 10 incorporated in a cavity of a Vanadate laser system 22. A gain media 24 is pumped through a dichroic mirror 26 by an output 28 of a fiber-coupled diode bar 30. Also included is an output coupler 32 and a focussing mirror 34. Laser system 22 produces a mode- locked output 36.

It is contemplated that the quantum wells can be grown at low temperature to produce a short response time for the device. It is understood that, while the Group III-V material system GaAs/AlAs is described above for fabricating Bragg reflector 10, other material combinations may be selected from other semiconductor Group III-V systems such as GaAs/InGaAs, InGaAs/InGaAlAs, InGaAs/InAlAs, AlAs/AlGaAs, GaAsSb/GaAlAsSb and InGaAsP/InP. The layers may be lattice-matched to suitable GaAs or InP substrates. Mismatching is also contemplated wherein strained layers are grown over the substrate material. Additionally, other semiconductor compounds in Group 11-VI and Group IV may also be used.

The foregoing description of a preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in this art. It is intended that the scope of the invention be defined by the following claims and their equivalents.

What is claimed is: