Optical system for reduced SBS

Details Optical system for reduced SBS Optical system for reduced SBS

1999-11-22

2001-12-18

Chan, Jason (Department: 2633)

Optical: systems and elements

Deflection using a moving element

Using a periodically moving element

C359S199200, C359S199200, C359S199200, C359S199200, C372S050121, C372S096000

Reexamination Certificate

active

06331908

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to laser transmitters and, more particularly, to broadband, tunable semiconductor laser transmitters which reduce stimulated Brillouin scattering (SBS) in fiber optic systems.
BACKGROUND OF THE INVENTION
SBS is one of many nonlinear phenomena which can adversely affect system performance in fiber optic systems. Brillouin scattering within a silica optical fiber results from photons being scattered by localized refractive index variations induced by acoustic (i.e., sound) waves. These refractive index variations are caused by acoustic vibrations in the silica lattice that makes up the fiber core. Furthermore, owing to the dependence of refractive index on light intensity in the nonlinear regime, the presence of intense light in the fiber will also induce lattice vibrations which, in turn, induce acoustic waves, that then scatter more light. When the SBS threshold power is exceeded (as low as about 10 mW per channel in some WDM systems), light from an intense forward propagating signal (e.g., the transmitted signal) can provide gain for (i.e., stimulate) a backward propagating signal (known as a Stokes signal). In this fashion, the Stokes signal can degrade the transmitted signal significantly.
Yet many applications require that the transmitted signal be launched at relatively high power, and anything, including SBS, which limits the maximum launch power presents a problem. For example, limiting the launch power reduces the allowable un-repeatered span length in fiber optic transmission systems, as well as the number of splits (or fanouts) which can be utilized in a fiber-based distribution system (e.g., a CATV system). One way to alleviate this problem is to increase the power at which the onset of SBS occurs (i.e., increase the SBS threshold). This threshold is arbitrarily defined as the level of launched optical power at which the power of the backward Stokes signal becomes equal to the power of the Rayleigh scattered signal; i.e., the total reflected power has doubled.
The prior art has devised numerous schemes for increasing the SBS threshold, but none is entirely satisfactory. Most of these schemes rely on the fact that the efficiency for SBS decreases as the linewidth of the transmitted signal source is increased. Consequently, artificially broadening the spectrum of that source via modulation serves to increase the SBS threshold. One approach calls for an external phase modulator to modulate the output of a laser transmitter, thereby broadening the spectrum of the transmitted signal by randomly changing its phase. A second approach utilizes wavelength dithering or detuning. A small specialized heating element is used to change the local laser temperature and thus its wavelength by a small amount. The frequency of the wavelength dither is on the order of a few kilohertz. However, these approaches require complex or high-voltage driving waveforms to broaden the spectrum. Alternatively, small-signal direct modulation of a DFB laser transmitter has also been suggested. But, when relatively large SBS thresholds are required, this approach results in substantial amplitude modulation (AM) which may degrade system performance. Another prior art approach, significantly different in that it does not involve artificially broadening the transmitted signal spectrum, suppresses SBS by applying duobinary modulation to the transmitted signal. Due to the absence of an optical carrier in the spectrum, the SBS threshold is increased compared to a conventional binary format. However, the duobinary format is not suitable for some systems applications.
Thus, a need remains in the art for an SBS reduction technique that can implemented with a simple, low voltage control signal, does not require special coding formats and has low levels of residual AM.
SUMMARY OF THE INVENTION
This need is addressed in a broadband, tunable laser which includes an intracavity, integrated composite reflector to which a tuning voltage and a dither signal are applied. The tuning voltage provides coarse tuning of the center wavelength, whereas the dither signal provides for SBS reduction. In one embodiment the dither signal comprises a sinusoidal waveform; in another it comprises a triangular waveform.
The tunable laser provides an optical output signal in any one of N different longitudinal modes at wavelengths corresponding, for example, to the N channels of a WDM system. Illustratively, the laser comprises an MQW active region, a DFB region for selecting the nominal wavelength of stimulated emission (i.e., laser light) generated by the active region, and a first waveguide optically coupled to the active region to permit egress of the laser output signal. The tunable laser is characterized in that a composite second reflector is coupled to one end of the first waveguide so as to form a cavity resonator with the DFB region. In accordance with one aspect of our invention, the second reflector includes an MQW second region optically coupled to the MQW active region, a second waveguide having one end optically coupled to the first waveguide, and a high reflectivity dielectric layer disposed at the other end of the second waveguide. In order to tune the center wavelength of the laser light, a forward bias tuning voltage is applied to the MQW second region to induce changes in refractive index through the plasma effect. In order to reduce SBS, the dither signal is also applied to the MQW second region to broaden the spectrum of the output signal.


REFERENCES:
patent: 5329396 (1994-07-01), Fishman et al.
patent: 5359450 (1994-10-01), Ramachandran et al.
patent: 5420868 (1995-05-01), Chraplyvy et al.
patent: 5473625 (1995-12-01), Hansen et al.
patent: 5477368 (1995-12-01), Eskildsen et al.
patent: 5566381 (1996-10-01), Korotky
patent: 5606573 (1997-02-01), Tsang
patent: 5991061 (1999-11-01), Adams et al.
patent: 5991323 (1999-11-01), Adams et al.
patent: 6081361 (2000-06-01), Adams et al.
patent: 0169567 (1986-01-01), None
patent: 0189252 (1986-07-01), None
patent: 762577A1 (1997-03-01), None
patent: 2309607 (1997-07-01), None
T. Numai, 1.5-&mgr;m Wavelength Tunable Phase-shift Controlled Distributed Feedback Laser, J. Lightwave Tech., vol. 10, No. 2, pp. 199-205 (1992).
W.T. Tsang et al., Control of Lasing Wavelength in Distributed Feedback Lasers by Angling the Active Stripe with Respect to the Grating, IEEE Photonics Tech. Lett., vol. 5, No. 9, pp. 978-980 (1993).
H. Nakajima et al., Absorption-controlled tunable DFB amplifier-filters, Electronic Lett., vol. 30, No. 16, pp. 1294-1296 (1994).
H. Hillmer et al., Tailored DFB Laser Properties by Individually Chirped Gratings Using Bent Waveguides, IEEE J. Selected Topics in Quantum Electronics, vol. 1, No. 2, pp. 356-362 (1995).
H. Nakajima et al., Very high speed wavelength switching capability Franz-Keldysh electoabsorption DFB lasers, OFC '96 Technical Digest, p. 276 (1996).
L. E. Adams et al., System Performance of High-Speed Broadband-Tunable Laser for Wavelength Conversion in WDM Networks, OFC '97, Postdeadline Paper No. PD11, pp. 1-4 (1997).
T. Tanbun-Ek et al., Tunable Electroabsorption Modulated Laser Integrated with a Bent Waveguide Distributed-Feedback Laser, IEEE Photonics Tech. Lett., vol. 9, No. 5, pp. 563-565(1997).
P.B. Hansen et al., SBS suppression using temperature wavelength-modulated laser diode with low residual AM, OFC 96 Technical Digest, Paper FR4, pp. 302-303 (1996).
T. Franck et al., Experimental Verification of SBS Suppression by Duobinary Modulation, ECOC 97, Conference Publication No. 448, pp. 71-74 (Sep. 22-25, 1997).
T. Franck et al., Novel Duobinary Transmitter, ECOC 97, Conference Publication No. 448, pp. 67-70 (Sep. 22-25, 1997).
T. Tanbun-Ek et al., Broad-Band Tunable Electroabsorption . . . , IEEE J. of Selected Topics in Quantum Electr., vol. 3, No. 3, pp. 960-966 (Jun., 1997).
G. C. Wilson et al., Predistortion of Electroabsorption Modulators for Analog . . . , vol. 15, No. 9, pp. 1654-1661 (Sep. 1997).
J. Salzman et al., Distributed Feedback Lasers with an S-Bent Wa

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