Coherent light generators – Particular beam control device – Optical output stabilization
Reexamination Certificate
2003-03-14
2004-10-12
Wong, Don (Department: 2828)
Coherent light generators
Particular beam control device
Optical output stabilization
C372S043010, C372S044010, C372S045013, C372S046012, C372S049010, C372S049010, C372S049010, C372S050121
Reexamination Certificate
active
06804272
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor self-pulsating laser diode and to a method for causing a semiconductor laser diode to output a plurality of light pulses in sequential light pulse cycles.
Semiconductor self-pulsating laser diodes are known. Commonly, such self-pulsating laser diodes output pulsed light of wavelengths of approximately 800 nm. Such laser diodes are commonly used for reading data from a disc, for example, a CD disc. It is widely believed that the self-pulsating behaviour of semiconductor laser diodes results from the provision of saturable absorber regions positioned adjacent an active wave guiding region of the laser diode such that light propagating in the active wave guiding region overlaps the saturable absorber region. The saturable absorber regions are formed by materials which have an intensity dependent absorption coefficient. As the active region is excited, the saturable absorber region absorbs light generated in the active wave guiding region, which in turn generates charge carriers in the saturable absorber region. The build-up of charge carriers in the saturable absorber region reduces the absorption coefficient of the material of the saturable absorber region, and thus reduces its capacity to absorb additional photons at that wavelength. Further excitation of the active wave guiding region causes the saturable absorber region to saturate, from which the term “saturable absorber” is derived. At saturation the absorber material of the saturable absorber region has a decreased loss, and thus the lasing condition for the laser diode is met and lasing light is emitted from the laser diode. The resulting emission of light from the laser diode depletes the charge carriers in the active region until lasing stops. Charge carriers generated in the saturable absorber region diffuse out of this region and return the absorber material to its original high absorption state, aiding quenching of the lasing emission and commencing the next cycle. In this manner a series of pulses of light are emitted from the laser diode in sequential cycles at a repetition rate which is predominantly determined by the carrier dynamics of the laser diode.
Many such self-pulsating laser diodes are described in the patent literature. U.S. Pat. Nos. 5,416,790 and 5,610,096 (both assigned to Sanyo Electric Co. Ltd.) disclose AlGaAs semiconductor lasers comprising a saturable optical absorbing layer having a band gap energy substantially equal to the energy corresponding to the lasing wavelength. U.S. Pat. No. 5,581,570 (assigned to Mitsubishi Denki Kabushiki Kaishi) discloses a semiconductor laser device comprising a saturable absorption region the function of which is the production of enhanced and pulsation oscillation at high power light output. In R. C. P. Hoskins, T. G. van de Roer, C. J. van der Poel, H. P. M. Ambrosius:
Applied Physics Letters
67, 1343, (1995), Hoskins et al teach that self-pulsation can be induced in broad area AlGaAs diode lasers by including an extra GaAs layer functioning as a saturable absorber.
In J. Buus:
IEEE Journal of Quantum Electronics
19,953, (1983), Buus has suggested that self-focusing, caused by the dependence of the refractive index on the carrier density, may contribute to self-pulsation in GaAs/GaAlAs devices which have no built-in wave guide. Buus discusses devices with relatively wide lasing stripes defined by the current injection regions which guide light by virtue of confining the gain to a stripe impressed along the light propagation axis of the laser. Any index guiding effects that may occur are fortuitous or incidental due to optically, photoelastically and thermally induced phenomena.
Considerable efforts have been made to develop a self-pulsating laser which would emit light at wavelengths shorter than 800 nm in order to satisfy demand for higher storage capacity capabilities. Such lasers are made from materials well-known to those skilled in the art and comprise suitable combinations of elements such as Indium, Gallium, Arsenic, Phosphorous, Aluminium, Nitrogen Cadmium, Zinc, Sulphur and Selenium for short wavelength (less than 700 nm) operation. Additionally, telecommunications applications could utilise robust self-pulsating laser devices made from these and other well-known elements which emit around longer wavelengths such as 0.98 &mgr;m, 1.3 &mgr;m or 1.5 &mgr;m.
Applying the saturable absorber approach to materials which emit light at 650 nm, Kidoguchi et al [I. Kidoguchi, H. adachi, T. Fukuhisa, M. Mannoh, A. Takamori:
Applied Physics Letters
68, 3543, (1996)] teach that self-pulsating AlGaInP lasers emitting light at a wavelength in the order of 650 nm may be fabricated by adopting a structure which has a highly doped saturable absorbing layer. Again using the saturable absorber approach U.S. Pat. No. 5,850,411 (assigned to SDL Inc.) discloses an AlGaInP/GaAs laser diode in which the active region is made up of quantum wells that are less than 5 nm thick such that quantum confinement of the charge carriers becomes significant. This facilitates operation with light emission of wavelength in the order of 620-650 nm. It is taught therein that self-pulsation may be obtained by the inclusion into such structures of a saturable absorber layer proximate to the active region.
Other devices emitting at 650 nm based on the incorporation of saturable absorbers are known. However, 650 nm lasers incorporating saturable absorbers have a number of disadvantages. The major disadvantage is the increased threshold current required to achieve lasing and to realise self-pulsation. This increase in threshold current arises from the increased loss introduced into the laser cavity by inclusion of the saturable absorption layer. An increase in the threshold current leads to an undesirable increase in heating in the device, which reduces the gain available in the laser due to its decrease with increasing temperature. Furthermore, in quantum well devices designed to emit at around 650 nm, or 1.3 to 1.5 &mgr;m for that matter, an increase in the threshold current required to operate the device causes an increase in leakage of charge carriers over the hetero-barriers of the structure. This in turn has a detrimental impact on the temperature and self-pulsation properties of the device.
There is therefore a need for a self-pulsating laser diode which overcomes the disadvantages of known self-pulsating laser diodes, and there is also a need for a method for causing a laser diode to output light pulses.
The present invention is directed towards providing such a self-pulsating laser and such a method.
SUMMARY OF THE INVENTION
According to the invention there is provided a semiconductor self-pulsating laser diode comprising a wave guiding layer, wherein the laser diode is configured so that when the laser diode is pumped,
(a) an active wave guiding region is defined in the wave guiding layer, the active wave guiding region comprising a pulsed light generating region in which pulsed light is guided during respective sequential light pulse cycles, the pulse light generating region extending longitudinally in the direction of pulsed light propagation, and an adjacent light propagating region in which light is propagated, and
(b) during each light pulse cycle the carrier density profile across the active wave guiding region progressively varies such that initially the carrier density in the pulse light generating region rises relative to the carrier density in the light propagating region until the difference between the refractive index of the pulse light generating region and the refractive index of the light propagating region is at its greatest, and the carrier density of the pulse light generating region reaches its lasing threshold value, thus causing lasing to commence in the active wave guiding region, and the lasing in the pulse light generating region progressively reduces the carrier density therein, which in turn progressively reduces the relative difference between the refractive index of t
Landais Pascal Michel
Lynch Stephen Anthony
McEvoy Paul
O'Gorman James Christopher
Flores-Ruiz Delma R.
Sughrue & Mion, PLLC
The Provost Fellows and Scholars of the College of the Holy and
Wong Don
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