Patent Application
20120163405
The invention relates to generally a coherent light source having low chirp. The invention has particular, but not exclusive, relevance to light sources for fiber optic communication systems.
Dispersion management is one of the key techniques for optical fiber communication, for example around the 1.5 micron telecommunications window. Dispersion is caused by optical signals with different wavelengths propagating at different speeds in the optical fiber. Therefore, an original optical pulse having components at multiple optical frequencies will spread while propagating through an optical fiber, resulting in distortion of the optical pulse or smearing of two optical pulses at the time of detection.
A single-mode distributed feedback semiconductor laser has a number of attractive properties as a coherent light source for optical communication, including a very narrow spectral linewidth in the order of 1 Megahertz. Although external modulation schemes have been employed, it is preferred to use direct current modulation since the external modulation schemes generally require higher voltages and increased device footprint. Direct current modulation has, however, the effect of introducing chirp both due to a difference in the lasing frequency at different injection current levels resulting from a variation in the optical refraction index (static or adiabatic chirp) and due to transient effects occurring at changes of injection current level (transient chirp). A typical laser diode may have a chirp factor of 100 MHz/mA, resulting in an optical spectrum of 3 Gigahertz under 30 mA direct current modulation. For analog optical communications, this introduces severe RF signal distortion.
Over communication links having a fixed distance, pre-distortion circuits may be used to compensate for dispersion. However, for low-cost communications it is preferred to have a single module operating over a range of distances (for example 0 to 20 km for FTTx systems) instead of having fixed length communication links. Accordingly, there is a desire for a laser diode with reduced chirp.
A disadvantage of reducing the chirp introduced by a coherent light source is that the reduced linewidth is an increase in interferometric intensity noise and nonlinear effects such as Stimulated Brillouin Scattering (SBS). In the article “A Method for Reducing Multipath Interference Noise” by S. L. Woodward and T. E. Darcie, IEEE Photonics Technology Letters, Vol. 6, No. 3, March 1994, it is proposed to reduced multipath interference intensity noise by dithering the laser frequency of a DFB laser diode by several Gigahertz at kilohertz frequencies (see also U.S. Pat. No. 5,373,385). The comparatively low modulation frequency allows the modulation to be achieved by temperature modulation as a result of varying the current supplied to a resistive heater formed on the DFB laser diode.
In the article “Single Contact Monolithically Integrated DFB Laser Amplifier” by R. T. Sahara et al., IEEE Photonics Technology Letters, Vol. 14, No. 7, July 2002, in order to achieve high-power operation it is proposed to integrate monolithically a distributed feedback (DFB) laser diode and an optical amplifier.
It is an object of the invention to provide a coherent light source having low-chirp properties.
This and other objects are provided by a coherent light source having a semiconductor laser resonator and an optical amplifier which amplifies coherent light emitted by the semiconductor laser resonator in response to current injection, in which the amount of current injected into the semiconductor laser is controlled for conformity with a chirp requirement of an optical communication system. The optical amplifier, which introduces minimal chirp, may be controlled to match an optical power requirement of the optical communication system.
This and other objects are also provided by a coherent light source having a semiconductor laser resonator and an optical amplifier which amplifies coherent light emitted by the semiconductor laser in response to current injection, in which a heater is provided to modulate the temperature of the semiconductor laser resonator. Such temperature modulation results in a corresponding variation of the laser wavelength, resulting in an increase in the linewidth of the emitted coherent light. This increase in the linewidth reduces multipath interference intensity and undesirable non-linear effects such as SBS.
This and other objects are further provided by a semiconductor laser having a monolithic gain region, having a first section forming a laser resonator and a second section forming an optical amplifier, and first and second electrodes arranged for injecting current into the first and second sections respectively. This facilitates the injection of a first current into the laser resonator to produce coherent light satisfying a desired chirp requirement, and a second current into the optical amplifier to satisfy an optical power requirement.
An embodiment of the invention provides a coherent light source which is well suited for an analog optical fiber communication system, such as CATV, in that it exhibits a dynamic bandwidth over 0-2 GHz with little variation in gain profile.
As shown in
In this embodiment, the length of the coherent light source is 750 microns. The semiconductor laser resonator 3 extends over half the length (i.e. 375 microns) of the coherent light source 1 and the semiconductor optical amplifier 7 extends along the other half of the length (i.e. 375 microns) of the coherent light source 1. It will be appreciated that the coherent light source 1 may have other lengths, and the ratio of the length of the laser resonator 3 relative to the optical amplifier 7 is a design choice.
The semiconductor optical amplifier 7 shows negligible adiabatic chirp (the dominant chirp for CATV and other analog communication systems) for RF modulation up to 1 Gigahertz. Accordingly, the current injected into the semiconductor optical amplifier causes negligible chirp while the coherent light source 1 still generates the necessary optical modulation index (OMI) for analog communication applications. In this embodiment, the current injected into the semiconductor laser resonator is controlled to achieve a chirp factor which matches a target chirp requirement for an optical communication system. The optical amplifier provides the required optical output power. It will be appreciated that the absolute and relative lengths of the semiconductor laser resonator 3 and the semiconductor optical amplifier 7, and the strength of the grating 5, can be adjusted to achieve the desired performance parameters.
While the coherent light source 1 of the first embodiment exhibits low chirp, the reduced linewidth may lead to unwanted interferometric intensity noise and second-order effects such as SBS. As shown in
In this embodiment, the resistive heater 23 is formed by a layer of Ti/NiCr/Pt. A drive circuit 27 supplies a drive signal to the resistive heater 23 which varies the temperature of the semiconductor laser resonator 3, thereby varying the laser wavelength. In particular, the variation of temperature introduces a thermal chirp typically with a frequency in the range of 10 to 100 kHz. This variation in the laser wavelength suppresses SBS and interferometric intensity noise without severely compromising the performance of, for example, CATV channels between 50 MHz and 1 GHz.
As discussed above, in the first embodiment a common electrode injects current both into the laser resonator 3 and the optical amplifier 7. As shown in
Providing separate electrodes 33a, 33b allows greater controllability of the optical properties of the coherent light source 31. In particular, by allowing different currents to be injected into the semiconductor laser resonator 3 and the semiconductor optical amplifier 7, a single device can be used to achieve many different combinations of chirp factor and optical power output. Alternatively, it may be desirable to supply a constant current to the semiconductor laser resonator 3 and a modulated current only to the semiconductor optical amplifier 7.
In the first to third embodiments, the semiconductor laser resonator 3 and the semiconductor optical amplifier 7 are monolithically integrated and share a common gain region. Such an arrangement is advantageous both with respect to device footprint and simplicity of driving. However, such monolithic integration is not essential. For example, the semiconductor laser could be coupled to a fiber amplifier.
The first to third embodiments are semiconductor devices. The composition of the semiconductors used will depend on the desired lasing wavelength, as is well known to those skilled in the art. Around 1550 nm, InP based systems using one or more of InGaAs, InGaAsP and AIGaInP may be used.
The coherent light sources discussed above are well suited to optical fiber communication systems, including analog systems such as CATV.
It will be appreciated that the above embodiments are described for exemplary purposes only, and many modifications will be apparent to a person of ordinary skill in the art.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 12632625 | Dec 2009 | US |
| Child | 13407925 | US |
