Optical transmitter

Details Optical transmitter Optical transmitter

1998-05-11

2001-11-20

Pascal, Leslie (Department: 2633)

Optical: systems and elements

Deflection using a moving element

Using a periodically moving element

C359S199200, C359S245000, C359S199200, C359S199200, C359S199200, C359S199200, C372S026000

Reexamination Certificate

active

06320688

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical transmitter, and in particular to a transmitter suitable for use in the transmission and distribution of analogue optical signals modulated at RF or microwave frequencies.
2. Description of Related Art
The low losses and EMI immunity associated with optical fibres, makes their use an attractive proposition, for example, in the distribution of signals to remote transmitter sites in a cellular radio system. The optical transmitters used hitherto with such optical fibre links have either been in the form of directly modulated laser diodes, or have comprised continuous wave lasers coupled with a separate electro-optic modulator such as a Mach-Zehnder intensity modulator or an electroabsorption modulator. Such optical transmitters suffer however, from a dynamic range which is significantly inferior to that of the electronic devices commonly used in cellular base stations. This has restricted the use of optical analogue links.
For a given received optical power, the dynamic range of analogue opticalfibre links is limited primarily by the linearity of the electrical to optical transfer characteristic of the optical transmitter. It has previously been proposed to use schemes such as electrical pre-distortion or optical feed-forward linearisation to increase the overall system dynamic range. It would be desirable however to improve the intrinsic source linearity. In the context of a source using a continuous wave laser followed by a Mach-Zehnder modulator, it has been proposed to use two modulators in cascade in order to improve linearity [Betts. G.E. IEEE Trans. Microwave Theory & Techniques, vol 42 no. 12 pp 2642-2649]. By feeding an RF drive signal to both modulators in a prescribed ratio, the second modulator corrects for the distortion of the first.
BRIEF SUMMARY OF THE INVENTION
According to a first aspect of the present invention, there is provided an optical transmitter comprising:
a) a directly modulated semiconductor laser including an input for a high frequency analogue electrical modulating signal; and
b) a non-linear optical intensity modulator which is connected in series with the optical output of the laser and which includes an input for a high frequency analogue electrical modulating signal corresponding to the signal applied to the laser, the modulator having a transfer characteristic arranged to cancel at least partially intermodulation distortion in the optical signal output by the laser.
The present invention provides a directly modulated laser source with an intrinsically improved linearity. This achieved by using a modulator following the directly modulated laser having distortion characteristics which tend to cancel the distortion produced by the laser.
Preferably the modulator is an electroabsorption modulator preferably integrated with the semiconductor laser. Preferably the laser is a multi-quantum well (MQW) distributed feedback (DFB) laser, and the modulator is an MQW electroabsorption modulator.
The preferred implementation of the present invention uses monolithically integrated components which together form a compact, single-chip linearised analogue source. This arrangement offers superior RF phase stability by comparison with implementations using discrete devices. Furthermore, the non-linearity of electro absorption modulators is highly controllable, making them ideal for the task of linearising a DFB laser.
Alternatively, the electro-optic modulator may be a Mach-Zehnder device which may be a planar device.
The modulator may have a predetermined characteristic chosen to be complementary to the expected characteristics of the modulated source. Preferably however the modulator is actively controlled to linearise the output signal. Preferably the system comprises a detector for monitoring the output of the transmitter, and means for supplying a control input to the modulator dependent on the output of the detector. The detector may be arranged to detect, for example, the third harmonic of the output signal, and to drive the modulator in a feedback loop to minimise this output.


REFERENCES:
patent: 5165105 (1992-11-01), Haase et al.
patent: 5166509 (1992-11-01), Curran
patent: 5278690 (1994-01-01), Vella-Coleiro
patent: 5359679 (1994-10-01), Tanaka et al.
patent: 5495359 (1996-02-01), Gertel et al.
patent: 5625636 (1997-04-01), Bryan et al.
patent: 5659565 (1997-08-01), Kitamura
patent: 5784188 (1998-07-01), Nakamura et al.
patent: 5793782 (1998-08-01), Meyrueix
patent: 6008926 (1999-12-01), Moodie et al.
patent: 6097525 (2000-08-01), Ono et al.
patent: 0 407 919 (1991-01-01), None
patent: 0 437 836 A2 (1991-07-01), None
patent: 0 484 791 (1992-05-01), None
patent: 0 493 817 (1992-07-01), None
patent: 0 658 016 (1995-06-01), None
patent: WO 92 13291 (1992-08-01), None
Morito et al, “High Power Modulator Integrated DFB Laser Incorporating Strain-Compensated/MQW and Graded SCH Modulator for 10Gbit/s Transmission”, Electronics Letters, Jun. 8, 1995, vol. 31, No. 1: pp. 975-976.
Guy et al, “Low Repetition Rate Master Source for Optical Processing in Ultrahigh-Speed OTDM Networks”, Electronics Letters, Sep. 28, 1995, vol. 31, No. 20, pp. 1767-1769.
Ellis et al, “40 Gbit/s Transmission Over 202 km of Standard Fibre Using Midspan Spectral Inversion”, Electronics Letters, Feb. 16, 1995, vol. 31, No. 4, pp. 299-301.
Moodie et al, “Generation of 6.3ps Optical Pulses at a 10GHz Repetition Rate Using a Packaged Electroabsorption Modulator and Dispersion Compensating Fibre”, Electronics Letters, Sep. 29, 1994, vol. 30, No. 20, pp. 1700-1701.
Devaux et al, “Experimental Optimisation of MQW Electroabsorption Modulators with Up to 40 GHz Bandwidths”, Electronics Letters, Aug. 4, 1994, vol. 30, No. 16, pp. 1347-1348.
Koren et al, “Low-Loss InGaAs/InP Multiple Quantum Well Optical Electroabsorption Waveguide Modulator”, Appl. Phys. Lett. 51 (15), Oct. 12, 1987, pp. 1132-1134.
Tanaka et al, “5-Gb/s Performance of Integrated Light Source Consisting of/4-Shifted DFB Laser and EA Modulator with SI InP BH Structure”, Journal of Lightwave Technology, vol. 8, No. 9, Sep. 1990, pp. 1357-1362.
Moodie et al, “Multiquantum Well Electroabsorption Modulators for 80 Gbit/s OTDM Systems”, Electronic Letters, Aug. 3, 1995, vol. 31, No. 16, pp. 1370-1371.
Moodie et al, “Discrete Electroabsorption Modulators with Enhanced Modulation Depth”, Journal of Lightwave Technology, vol. 14, No. 9, Sep. 1996, pp. 2035-2043.
Devaux et al, “High-Speed, InGaAsP/InP Multiple Quantum Well, 1.55m Singlemode Modulator”, Electronics Letters, 27 (1991) Oct. 10, No. 21, Stevenage, Herts., GB, pp. 1926-1927.
IEEE Photonics Technology Letters, vol. 7, No. 10, Oct. 1995, New York, US, pp. 1154-1156, Wilson et al, “Integrated Electrabsorption Modulator/DBR Laser Linearized by RF Current Modulation”.
Journal of Lighwave Technology, vol. 13, No. 8, Aug. 1995, New York, US, pp. 1606-1612, Iwai et al, “Signal Distortion and Noise in AM-SCM Transmission Systems Employing the Feedforward Linearized MQW-EA External Modulator”.
IEEE Photonics Technology Letters, vol. 3, No. 9, Sep. 1991, New York, US, pp. 844-846, Suto et al, “Intermodulation Distortion in 48 TV Channel FM-FDM Optical Transmission”.
IEEE Photonics Technology Letters, vol. 6, No. 9, Sep. 1994, New York, US, pp. 1150-115, Wu et al, “A Fiber Distribution System for Microcellular Radio”.

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