Coherent light generators – Particular beam control device – Optical output stabilization
Reexamination Certificate
1998-11-30
2001-07-24
Arroyo, Teresa M. (Department: 2881)
Coherent light generators
Particular beam control device
Optical output stabilization
C372S075000
Reexamination Certificate
active
06266351
ABSTRACT:
FIELD OF THE INVENTION
The invention is directed to a generator for producing a low-noise, high-frequency signal, a monomode fob laser being connected via an optical isolator to a second laser, and light produced by the first laser being injected into the second laser.
RELATED TECHNOLOGY
Microwave-based radio-communication systems require carrier frequencies in the range of 10 GHz and higher. Radio-communication systems of this kind are used, for example, to supplement permanently cabled subscriber lines in the form of wireless local loops (WLL). These systems are suited for providing customers with new communications services quickly, flexibly and at low cost.
Various methods for producing high-frequency carriers have become known For example, L. Goldberg, A. M. Yurek, H. F. Taylor, and J. F. Weller discuss locking a Fabry-Perot laser onto an injected sideband in “35 GHz Microwave Signal Generation with an Injection-Locked Laser Diode”, Electron. Lett., vol, 21, pp 630631, 1985. Also, D. Wake, C. R. Lima and P. A. Davies describe a high-frequency, driven dual-mode laser having distributed feedback in “Optical Generation of Millimeter-Wave Signals for Fiber-Radio Systems using a Dual-Mode DFB Semiconductor Lascr”, IEEE Trans. Microwave Theory and Techn., vol 43, pp. 2270-2276, 1995. Also known from D. Novak, Z. Ahnmed, R. B. Waterhouse and R. S. Tucker in “Signal Generation Using Pulsed Semiconductor Lasers for Application in Millimeter-Wave Wireless Links”, IEEE Trans. Microwave Theory and Techn., vol 43, pp 2257-2262, 1995 is an optically filtered mode locking of laser diodes.
In addition, D. Trommer, R. Kaiser, R. Stenzel, H. Heidrich in “Multi-Purpose Laser/Coinbiner PIC Based in InP”, Proc. ECOC '95, paper Mo.B.4.2, pp. 83-86, 1995, describe superpositioning two independent laser lines.
Various drawbacks are associated with the known methods Either they require additional electrical microwave injection or they are too noisy for use in radio-communication systems. Only when it comes to the subject matter of the last-named publication is a continuous tuning of the generated optical microwave frequencies possible. On the other hand, however, the noise is quite intense with this method.
A device is also known from WO 93116514 A1 for changing the frequency of light signals, whereby a source light signal is supplied to an input of a self-pulsing laser diode. In this context, the frequency of the laser diode matches that of the source signal ox of a harmonic of the same. The purpose of the arrangement is to produce different modulation frequencies of the light emitted by the laser.
SUMMARY OF THE INVENTION
An object of the present invention, therefore, is to devise a generator for producing a carrier or other high-frequency signal that is substantially low-noise, with the least possible outlay.
The present invention provides a monomode first laser is connected via an optical isolator to a second laser so as to permit light generated by the first laser to be injected into the second laser. The difference in the frequencies of the lasers and the intensity of the injected light is selected so as to prevent the second laser from locking onto the free-running frequency of the first laser, and so as to ensure that the second laser oscillates at a frequency that differs slightly from the free-running frequency and that its output signal exhibits a substantial modulation depth, with a modulation frequency of about the difference in the frequency of the two free-running lasers.
In one advantageous embodiment of the generator according to the present invention, the firs laser is a laser having an external resonator, or external cavity laser (ECL). This enables the generated frequency to be tuned effectively. For applications where such a tuning capability, in particular a large tuning range, is not required, a different laser, for example a laser having distributed feedback, would also be suited for use as the first laser and is within the scope of the present invention.
Another advantageous embodiment of the generator of the present invention provides for the second laser to be a laser having distributed feedback (DFB).
To generate a carrier frequency for microwave radiocommunication systems, a further development of the generator according to the present invention provides for a photoelectric transducer to be connected to the output of the second laser, the output of the second laser and the input of the photoelectric transducer preferably being linked via an optical isolator.
In this specific embodiment of the generator according to the present invention, the carrier modulation may be performed on the electrical level. Provision may also be made, however, for an optical modulator to be connected in series to the photoelectric transducer. As a result, the already modulated microwave signal is able to be carried over large distances, with low loss, using an optical waveguide, and not be converted into an electrical signal until the point of radiation.
Another field of application of the generator according to the present invention is producing extremely high frequency signals to be modulated, which are transmitted optically. To this end, the design of the generator according to the present invention may be such that the first laser is modulated with a frequency corresponding to the frequency of the signal, the second laser is locked onto a sideband produced by the modulation, and the output signal of the second laser is able to be modulated with a digital modulation signal, whose clock pulse corresponds to the frequency used to modulate the first laser. The output signal from the second laser is thus synchronized to the frequency at which the first laser is modulated. Provision is preferably be made in his context for he modulated output signal of the second laser to be able to be fed to an optical transmission device
According to another embodiment of the present invention, it is possible to modulate the generated signal by modulating the output signal from the first laser, using a signal fed to it, so that the generated signal is also modulated using the supplied signal The modulation of the output signal from the first laser may also be performed in addition to the modulation using a single sideband frequency. This embodiment may be designed with the first laser being modulated directly or with an external optical modulator being provided for modulation purposes. The modulation may be performed using both analog as well as digital signals.
Alternatively, to generate extremely high-frequency optical signals, provision can bc made for the output signal from the second laser to be able to be fed to an optical modulator and to a clock-pulse generator for providing a digital modulation signal for the optical modulator.
The present invention makes advantageous use of a property of the second laser, namely the property of producing an additional frequency besides a frequency corresponding substantially to the natural frequency of the second laser. This additional frequency arises when the differential frequency of the two lasers is selected to be greater than the locking range. This is caused by a change in (pulling of) of the mean optical frequency the field of the second laser, produced by injection. The output signal from the second laser can show oscillation frequencies of from 7 GHz to over 2 THz, both for positive as well as negative frequency differences. In practical applications, however, only frequencies of up to 45 GHz are measured, depending on the bandwidth of a connected photoelectric transducer. The line width at the (−3 dB) points is below 100 kHz when an ECL is used as a first laser. Using an interferometer and based on the generation of the second harmonic (SHG), frequencies of up to 2 THz having a modulation index of 90% have been measured.
BRIEF DESCRIPTION OF THE DRAWINGS
Exemplary embodiments of the invention are elucidated below on the basis of the drawings, in which:
FIG. 1
shows an exemplary embodiment of the present invention designed for the generation
Burkhard Herbert
Schoell Hansjoerg
Tamburrini Mario
Arroyo Teresa M.
Deutsche Telekom AG
Inzirillo Gioacchino
Kenyon & Kenyon
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