Coherent light generators – Particular active media – Semiconductor
Patent
1994-05-06
1996-08-06
Bovernick, Rodney B.
Coherent light generators
Particular active media
Semiconductor
372 47, 372 96, H01S 318
Patent
active
055441890
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of Invention
This invention relates to a semiconductor laser device to be suitably used as transmission light source in the field of optical telecommunications and optical instrumentation.
2. Prior Art
Data transmission systems having a transmission rate of 10 Gb/sec. or so have been proposed for large capacity trunk lines of the coming generation in the field of optical telecommunications. In this connection, efforts have been paid to exploit the potential of soliton transmission as it serves for optical data transmission without affecting the dispersion characteristics of the optical fibers involved.
Semiconductor laser devices to be used for soliton transmission are required to show a high speed modulation capability for frequencies of 20 GHz and above in order to generate ultrashort pulses of light with a pulse width not greater than 1.times.10.sup.-12 sec. (1 pico-sec.).
Paper 1 listed below discusses some of the results of theoretical researches made on the properties of semiconductor laser devices concerning light wave modulation, using rate equations [1a] and [1b] below for photon density S and carrier density N in an active layer. and
The modulation response of a semiconductor laser device can be determined by solving rate equations [1a] and [1b] and its highest modulable cutoff frequency is obtained by using equation [2] below. gain and the saturation constant of gain G(S, N) above when it is expressed in terms of threshold carrier density N.sub.t as shown by equation [4] below.
By studying the above equations, it is evident that the value of K needs to be reduced in order to expand the modulation bandwidth of a semiconductor laser device by increasing frequency fm of the semiconductor laser device and, by turn, it is necessary to reduce p, increase g.sub.o or reduce .epsilon. in order to reduce K.
Known techniques for reducing .tau..sub.p include reducing the length of the cavity and/or the reflectivity of the facet of the semiconductor laser cavity, whereas it is known that g.sub.o can be increased by doping the active layer to turn it into a p-type layer.
However, the values of g.sub.o and .epsilon. are invariable and .tau..sub.p can easily encounter a lower limit because it is correlated with the threshold current density when the semiconductor laser device is prepared by using a single and same semiconductor material such as GaInAsP. Thus, the above identified techniques are not feasible to reduce the value of K for such a semiconductor laser device.
A promising technique is the use of a multiple or single quantum well structure for the active layer in order to multiply the currently available value of g.sub.o by two to three times.
On the other hand, it, is pointed out in Paper 2 below that a quantum well structure can degrade the modulation response of a semiconductor laser device and therefore the time required for carriers to be injected into a quantum well structure is inevitably prolonged because of the specific properties of the quantum well structure. Paris, France, September 1991.
Now, some of the problems pointed out in Paper 2 above will be summarized below.
The rate equations for a semiconductor laser device having a quantum well structure can be obtained by modifying equations [1a] and [1b] as shown below. /.tau..sub.e (V.sub.w /V.sub.b) [5b] (V.sub.b /V.sub.w)-G(S,N.sub.w) [5c] well layer, captured by the quantum well layer (which depends on the time required for carriers to run through the barrier layer or the optical confinement layer) and emitted from the quantum well layer, so-called thermionic emission.
A conventional semiconductor laser device having a multiple quantum well structure modifies the density of carriers .DELTA.J.sub.b injected into the barrier layer by reducing the density of carriers injected into the quantum well layer .DELTA.J.sub.w to zero but the modulation response of a semiconductor laser device, or the response of photon density S to .DELTA.J.sub.b, obtained by using such a modulation technique of modulating the c
REFERENCES:
patent: 4166278 (1979-08-01), Suzaki et al.
patent: 4901327 (1990-02-01), Bradley
patent: 5343487 (1994-08-01), Scott et al.
Bovernick Rodney B.
McNutt Robert
The Furukawa Electric Co. Ltd.
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