Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
1998-12-15
2003-11-18
Ip, Paul (Department: 2877)
Coherent light generators
Particular active media
Semiconductor
C372S022000, C372S096000, C372S102000, C372S025000
Reexamination Certificate
active
06650673
ABSTRACT:
FIELD OF INVENTION
The invention relates to generation of short optical pulses with particular application to transmission of data.
BACKGROUND OF THE INVENTION
Ultra high speed time domain multiplexing (TDM) optical transmission systems in optical fibers require compact light emitting sources capable of generating optical short pulse trains in a picosecond/sub-picosecond range. General requirements for short pulse sources such as soliton sources are narrow pulse width, low time jitter and a continuously tunable repetition rate. For practical fiber optic systems, there are additional requirements of long-term reliability, small size and easy data encoding in the system application.
There are several known methods to generate short optical pulse trains. Complicated passive or active mode locking techniques are available for high speed optical pulse generation where pulses are generated at a fixed repetition rate determined by the roundtrip time of the laser resonator, e.g. D. J. Derickson et al., “Short pulse generation using multisegment mode-locked semiconductor lasers,” IEEE J. Quantum Electron., Vol. 28, pp. 2186-2202, 1992. These techniques are sensitive to phase matching conditions and therefore difficult to build and maintain. Another method is gain switching of lasers which suffer from high time jitter. Pulse generation at repetition rates over 50 GHz is extremely difficult to achieve in this method because of limitations of the device modulation bandwidth and radio frequency supply as described, e.g. in publication by A. G. Weber, W. Ronghan, E. H. Bottcher, M. Schell and D. Bimberg, “Measurement and simulation of the turn-on delay time jitter in gain-switched semiconductor lasers,” IEEE J. Quantum Electron., Vol. 28, pp. 441-445, 1992.
High repetition rate optical pulses can also be generated using a dual wavelength light source as described, e.g. in publication P. V. Mamyshev, S. V. Chernikov and E. M. Dianov, “Generation of Fundamental soliton trains for high-bit-rate optical fiber communication lines,” IEEE J. Quantum Electron., vol. 27, pp. 2347-2355, 1991. Two wavelengths emitted by two lasers are mixed to form a high frequency sinusoidal signal which is sent though an optical combiner and optical amplifier followed by a nonlinear fiber. As a result the sinusoidal signal is compressed into a train of optical pulses. The common approach of dual wavelength light sources is to use two discrete lasers, which is complex and suffers from the long term stability issue. Dual wavelength operation can also be accomplished by selecting the appropriate phase modulation sidebands from an externally phase modulated light source, e.g. P. V. Mamyshev, “Dual-wavelength source of high-repetition-rate, transform-limited optical pulses for soliton transmission” Opt. Lett., Vol. 19, pp. 2074-2076, 1994. This method requires high frequency modulation and optical filters. In order to make this method more practical two solitary laser diodes are usually used to generate the sinusoidal beat signal. Unfortunately, frequency variations of each laser are subject to both thermal and mechanical fluctuations which result in beat signal frequency fluctuations. Phase noise of both lasers also contribute to the jitter of the beat signal significantly. The control of the polarization from each laser output and the effort to align and maintain them is also a practical issue that decreases the system performance. Therefore, the resulting signal performance is not satisfactory and practical use of such a configuration in commercial ultra high speed applications is in question.
Accordingly, there is a need in the industry for a practical, compact and reliable optical source of continuously tuning high repetition rate short optical pulses which is suitable for optical transmission systems and high speed optical signal processing.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an optical pulse source which avoids the afore-mentioned problems.
Thus, according to one aspect of the present invention there is provided an optical pulse source, comprising:
a first single mode DFB semiconductor laser having a first grating for generating light at a first frequency;
a second single mode DFB semiconductor laser having a second grating for generating light at a second frequency;
the lasers having a common active medium and shared optical path, the lasers providing mutual light injection into each other resulting in generation of a beat signal at a difference frequency of two lasers;
an optical compressor disposed to receive the beat signal and compressing the pulse duration of the signal, thus forming a train of short optical pulses having a pre-determined duration and a repetition rate.
The source may further include a saturable absorber disposed to receive the beat signal before it is sent to the optical compressor. The absorber provides an initial time compression of the signal, thus transforming the beat signal into an initial train of optical pulses. Additionally, an optical amplifier may be used for amplification the beat signal or the initial train of pulses, e.g. including an erbium doped fiber. The optical compressor may include a dispersion decreasing fiber, a dispersion shifted fiber and/or an external erbium doped fiber amplifier. The source may further include means for data encoding into the train of short pulses, e.g. an optical modulator operating at a speed determined by the repetition rate. A typical range of the repetition rates is from about several tens GHz to about several hundred GHz with a sub-range from about 25 GHz to about 80 GHz being of special importance for data transmission. A typical duration of pulses in the pulse train is within a picosecond/sub-picosecond range.
A source includes gain coupled DFB lasers, or alternatively it may include loss coupled DFB lasers. Preferably, the active medium of the lasers includes a multiple quantum well structure. Advantageously, the first and second gratings in the first and second lasers are formed by etching grooves directly through the multiple quantum well structure. Beneficially, each grating has a period comprising a first section and a second section with substantially all quantum wells being etched away from the second section, thus providing no substantial photon emission in the second section and ensuring no substantial interaction between the lasers. A source may further have means for stabilizing the frequency of one of the first and second lasers, or means for stabilizing frequencies of both lasers. Means for tuning frequencies of the first and second lasers may be provided additionally. To ensure reliable and accurate mode, low frequency modulation of light generated by one of the first and second lasers, may be provided. Alternatively, light generated by the lasers may be modulated simultaneously. Beneficially, the modulation is provided at a frequency which is subharmonic to the beat frequency.
In the first embodiment a source includes first and second gratings which have same periods, and it is arranged that lasers generate light at the different sides of stopband.
In the second embodiment a source includes first and second gratings which have same periods, and it is arranged that lasers generate light at the same side of stopband. The difference between the first and second frequencies is provided by different current injection into the first and second lasers, or by difference in temperature control of the first and second lasers. Alternatively it may be provided by different width of the active medium in the first and second lasers.
In the third embodiment a source includes the first and second gratings which have different periods.
It is arranged that the frequency of one of the lasers which is remote from an output facet does not fall within a stopband of the other laser which is closer to the output facet so that light emitted by the remote laser can pass through the shared optical path to the output facet.
In the fourth embodiment, instead of pumping the active medium of the source by curren
Hong Jin
Hui Rongqing
O'Sullivan Maurice S.
Bookham Technology PLC
Ip Paul
Lahive & Cockfield LLP
Rodriguez Armando
LandOfFree
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