Coherent light generators – Particular beam control device – Q-switch
Reexamination Certificate
1998-03-23
2001-03-27
Arroyo, Teresa M. (Department: 2881)
Coherent light generators
Particular beam control device
Q-switch
C372S025000
Reexamination Certificate
active
06208672
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical pulse source. Such a source might be used, for example, in a transmitter for a broadband optical network or in an optical interconnect.
2. Related Art
Broadband optical networks are expected to operate at data transmission rates of 100 Gbit/s and beyond. Operation at such rates places heavy demands on the optical sources used. The repetition rate required of the pulse source may be less than the full bit rate, since the datastream at the full bit rate will typically be generated by multiplexing the optical source in the time domain. However, for this to be possible, the pulses output by the source need to be of short duration and desirably should have low temporal jitter.
SUMMARY OF THE INVENTION
According to a first aspect of the present invention there is provided an optical pulse source comprising:
a) a gain-switched semiconductor laser diode;
b) a continuous wave light source which is coupled to the optical cavity of the semiconductor laser diode;
c) an electro-optic amplitude modulator which is connected to the optical output of the semiconductor laser diode; and
d) control inputs, located respectively on the semiconductor laser diode and on the electro-optic modulator, for applying synchronised modulating signals to the semiconductor laser diode and to the electro-optic modulator.
The present invention uses in combination a laser diode, injection of a cw beam into the laser diode cavity, and gating of the laser diode output using a synchronously driven amplitude modulator. The present inventors have found that this combination provides a pulse source markedly superior to sources known hitherto. A source embodying the present invention has produced pulses of 4 ps duration with temporal jitter of only 0.6 ps. Such pulses are suitable for use at bit rates as high as 100 Gbit/s. By contrast, previously proposed directly modulated sources have not been capable of operation beyond 40 Gbit/s. Examples of such prior art systems include that disclosed in the paper by M J Guy et al “Low Repetition Rate Master Source for Optical Processing in Ultra High-Speed OTDM Networks” ELECTRONICS LETTERS, Sept. 28, 1995, Vol 31, Number 20 pp 1767-1768. This discloses a system using a DFB (distributed feedback) SLD and a synchronously driven electro-absorption modulator connected to the output of the laser. This system improves the pulse shape by reducing the pulse pedestal, but suffers from performance limitations associated with the interaction between the modulator and the jitter in the pulses from the semiconductor laser diode.
It has also been proposed previously to use injection of a cw beam into a directly modulated laser diode. An example of a source using cw injection is disclosed in “A High Speed Broadcast and Select TDMA Network Using Optical Demultiplexing”, L P Barry et al, Proceedings 21st European Conference on Optical Communications, ECOC '95 pp 1437-440. Injection of cw light tends to increase the pulse pedestal and so although the temporal jitter is reduced, the overall suitability of the pulse for use at high bit rates remains limited. The present inventors have found however that when cw injection and amplitude gating are used in combination, there is a mutual interaction between their modes of operation, so that the quality of the output pulse is markedly superior to that produced when either of these two techniques is used in isolation. The use of cw injection reduces the jitter in the signal input to the amplitude modulator and this eliminates many of the dispersion-related effects otherwise associated with the use of such a modulator. At the same time, the modulator largely eliminates the pedestal which would otherwise be present in the output pulse when cw injection is used.
Another advantage of the present invention is that the use of cw injection reduces the bias voltage required by the laser. This makes the source particularly suitable for use, for example, in the laser array of an optical interconnect such as that disclosed in U.S. Pat. No. 5,363,961 (Hamanaka). In such an array it is advantageous to reduce or eliminate the need for bias voltage, so as to reduce the overall power dissipation.
The semiconductor laser diode may be a distributed feedback semi-conductor laser diode (DFB-SLD) in which case preferably it has a buried heterojunction structure. Alternatively a ridge waveguide DFB-SLD might be used. The present invention is not however limited to the use of DFB lasers, and may also be implemented using, for example, Fabry-Perot Lasers (FPL), Vertical Cavity Surface Emitting Lasers, or Distributed Bragg Reflector Lasers. For some fields of use, the Fabry-Perot laser is particularly to be preferred. An FPL is capable of emitting at several discrete wavelengths. Conventionally, an external diffraction grating, which may be bulk optic or fibre-based, is used to select one of several possible wavelengths corresponding to different longitudinal modes of the laser. In systems embodying the present invention, the cw beam can be used in place of an external filter to select a given mode. The system then provides a pulse source tunable to different discrete wavelengths. Such a source is particularly suitable for systems using WDM (wavelength division multiplexing).
Preferably the electro-optic modulator is an electro-absorption modulator.
Electro-absorption modulators exhibit strongly non-linear variation of device optical absorption characteristics under an applied reversed bias voltage. This makes them particularly suitable for use as the synchronously driven amplitude modulator in a system embodying the present invention.
Preferably the pulse source further comprises optical pulse shaping means connected on the output side of the modulator. The pulse shaping means may comprise a length of normally dispersive fibre.
The shape of the pulses output from the source can be further enhanced using, for example, a length of normally dispersive fibre. Alternatively other pulse shaping means may be used. In particular, it may be advantageous to use a chirped grating in place of normally dispersive fibre. A suitable grating is disclosed, for example, in the paper by P. Gunning et al. at pp 1066-1067, ELECTRONICS LETTERS, Jun. 22, 1995, Vol. 31, No. 13 .
According to a second aspect of the present invention, there is provided a method of generating optical pulses comprising:
a) applying a first electrical modulating signal to a gate input of a gain-switched semiconductor laser diode;
b) coupling continuous wave light into an optical cavity of the semiconductor laser diode;
c) applying a second modulating signal, which is synchronised to the first electrical modulating signal, to a control input of an electro-optic amplitude modulator; and
d) passing pulses output from the semiconductor laser diode through the electro-optic amplitude modulator.
REFERENCES:
patent: 5519526 (1996-05-01), Chua
patent: 5739933 (1998-04-01), Dembeck
patent: 5778015 (1998-07-01), Gunning
patent: 5784185 (1998-07-01), Smith
patent: 5953138 (1999-09-01), Ellis
Electronics Letters, vol. 31, No. 20, Sep. 28, 1995, pp. 1767-1769, Guy et al, “Low Repetition Rate Master Source for Optical Processing in Ultrahigh-Speed OTDM Networks”.
IEICE Transactions on Electronics, vol. E78-C, No. 1, Jan. 1, 1995, pp. 50-54, wakita et al, “Short Optical Pulse Generation and Modulation by a Multi-Section MQW Modulator/DFB Laser Integrated Light Source”.
Proceedings of the International Semiconductor Laser Conference, Maui, Hawaii, Sep. 19-23, 1994, Institute of Electrical and Electronics Engineers, p. 43/44, Raybon et al, “Digitally Encoded Optical Pulse Generation from an Integrated DBR Laser-Modulator”.
Electronics Letters, vol. 32, No. 1, Jan. 4, 1996, p. 44/45, Seo et al, “Timing Jitter Reduction of Gain-Switched DFB Laser by External Injection-Seeding”.
Gunning Paul
Lucek Julian Kazimierz
Moodie David Graham
Nesset Derek
Russell Davey
Arroyo Teresa M.
British Telecommunications public limited company
Inzirillo Gioacchino
Nixon & Vanderhye P.C.
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