Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
1998-09-29
2001-04-10
Font, Frank G. (Department: 2877)
Coherent light generators
Particular active media
Semiconductor
C372S050121, C372S010000, C372S092000, C372S027000, C372S020000, C372S099000, C372S044010, C372S011000
Reexamination Certificate
active
06215805
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a Q-switched semiconductor laser consisting of at least two electrically separated but optically coupled resonators of which one is structured as a passive resonator and the other one is structured as an active resonator.
Since short and intensive laser pulses can be generated with such arrangements, Q-switched semiconductor lasers are used, among others, in the field of non-linear optics and communication/optical data transmission.
In a Q-switched laser, a high population inversion is built up by strong pumping, and leads to a high amplification. But high resonator losses initially prevent their lasing activity. Once the losses are overcome, a short light pulse of high impulse intensity will be emitted. One way of accomplishing Q-switching is to alter the external losses at the resonator reflectors; another way is initially to produce internal losses in the resonator which losses may subsequently be eliminated.
2. The Prior Art
In IEEE Photonics Technology Letters, Vol. 7, No. 10, October 1995, pp. 1125-1127, there is described a dual-section DFB laser one section of which is initially operated as an absorber and which is thereafter made transparent by current injection. To this end, the net amplification in the resonator is modulated. Compared to laser modulation by pumping current, this arrangement yields improvements in respect of modulation currents and in the chirp. Yet high frequency laser modulation with low currents or voltage ranges have thus far not been realized.
U.S. Pat. No. 4,982,405 describes a Q-switched laser having two optically coupled resonators. As there described, Q-modulation is accomplished by detuning the two resonators.
The state of the art upon which the invention is based is described in IEEE Journal of Quantum Electronics 30 (1994), May, No. 5, pp. 1204-1211. The described Q-switched semiconductor laser consists of two optically coupled resonators one of which is structured as a passive resonator and the other of which is structured as an active resonator. Both resonators may be electrically switched (electrodes on the reflectors, common center electrode for both reflectors), whereby pumping of the active section (causing amplification) and tuning of the refractive index of the passive section may be carried out in separate operations. In this solution, too, the power modulation referred to as Q-modulation is accomplished by detuning of the two resonators. In this arrangement, the range having a variable refractive index, together with the two Bragg reflectors embracing this range, acts as a variable reflector or as a tunable etalon. DBR reflectors are used as resonator reflectors because of their high and substantially constant reflectivity within the stop band. In this arrangement, the DBR structure at the laser wavelength acts exclusively as a highly reflective mirror. While these solutions known from the state of the art permit high-frequency Q-modulation of the lasers, they do so at a low modulation range.
OBJECT OF THE INVENTION
It is, therefore, the object of the invention to provide a Q-switched laser which makes high frequency laser modulation possible at low currents or voltage ranges, as the case may be.
BRIEF SUMMARY OF PREFERRED EMBODIMENTS
The object is accomplished by the fact that in a Q-switched semiconductor laser of the kind referred to above the two optically coupled resonators for laser mode selection are provided with differently structured mode combs which overlap spectrally such that at least one defined laser wavelength is selected, that at least one resonator reflector of the two optically coupled resonators is structured as a reflector with a strongly dispersive reflection characteristic in the range of the wavelength selected by the two optically coupled resonators, that means is provided for electrically or optically tuning the refractive index of the dispersive reflector and/or of the passive resonator so that spectral control of the laser mode is accomplished relative to the dispersive reflector.
With this arrangement, the reflectivity for the laser wavelength may be set such that at a constant effective amplification by electrical modulation of the reflectivity of the strongly dispersive resonator mirror, the lasing threshold may be lowered or raised such that the semiconductor laser may thereby by switched on or off.
A similar arrangement has previously been described in IEEE Photonics Technology Letters, Vol. 8, No. 1, January 1990, pp. 28-20. That multi-section laser is, however, operated in a different manner and serves a function different from the one of the present solution. Thus, in that self-pulsating multi-section laser, the active layer is pumped so strongly, i.e., it is switched at so high a current, that the active resonator is activated as a laser on its own. In contradistinction to the operation of the laser in accordance with the invention, the currents in the passive resonator and in the reflector are fixed at such values that noise of the charge carrier density in the active resonator which, in turn, leads to noise in of the laser wavelength, is stepped up by the wavelength dependent feedback from the reflector such that oscillations of laser power, wavelength and charge carrier density occur in the laser section.
In respect of the function of the Q-switched semiconductor laser in accordance with the invention, the presence of two optically coupled resonators having two mode combs which spectrally overlap such that a defined laser wavelength is selected—similar to the Nonius principle known from mechanics—is critical for the reason that laser emission would, because of the many possible modes in a simple semiconductor laser, not otherwise be supported by the desired mode. Instead, competing modes yielding higher and non-dispersive reflection would dominate the laser activity.
To structure one of the resonators as a passive resonator is important because its refractive index may be unequivocally set or adjusted externally while in an active resonator charge carrier density and, hence, refractive index and wavelength are additionally dependent upon the photon density. All in all, therefore, a laser mode is selected by the dual resonator, and the wavelength is substantially fixed by the passive resonator. This wavelength is, however, adjustable within a certain wavelength range by electrically switching of the passive resonator. A substantially spectral match of the wavelength with the strong dispersion in the reflector characteristic may be realized during fabrication. Fine tuning is accomplished by electrical offset when switching the passive resonator or reflector.
The actual modulation is then carried out by electrical switching (applied electrical field or charge carrier injection) of the refractive index of the passive resonator or by electrical switching of the reflector. The laser threshold may be very efficiently modulated, and the laser may thus be switched on and off without modulating the net amplification, even by very small changes of the spectral correlation of laser wavelength relative to the spectrally steep reflection flank of the reflector.
High frequency modulations are possible with the arrangement in accordance with the invention since the control signals require small ranges only. By comparison to the power modulation by laser current modulation a much small chirp (wavelength modulation) is obtained as the charge carrier density in the active resonator requires no modulation and since, moreover, the wavelength is fixed by the passive resonator.
Further advantageous and useful details of the invention are set forth in the subclaims. Among these are, in particular, the structure of the resonator mirrors and of the two optically coupled resonators.
This embodiments provide for structuring the strongly reflective resonator as a DFB or DBR grid which may be tuned relative to the refractive index, are as a narrow band interference filter on a laser facet.
In further embodiments, the reflector is provided w
Moehrle Martin
Sartorius Bernd
Flores Ruiz Delma R.
Font Frank G.
Heinrich-Hertz-Institut fuer Nachrichtentechnik Berlin GmbH
Hormann Karl
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