Optical amplifier

Abstract: The amplifier (1) is made up of two active fibre or guide amplifiers (2, 3) placed respectively before and after a gain clamped semiconductor amplifier (4), brought to stimulated emission conditions, which acts as pump radiation source for the two active fibre or guide amplifiers (2, 3).

Description: The invention described herein concerns optical communications systems and more particularly relates to an optical amplifier for use in such systems. In optical communications systems, two types of optical amplifiers are generally used: semiconductor amplifiers or active optical fibre (or in general active optical guide) amplifiers. Semiconductor amplifiers are devices of which the structure is essentially analogous to that of lasers and which are biased below the stimulated emission threshold, to avoid starting of laser oscillations, and above transparency, to exploit the amplification characteristics of the active material they are composed of. A problem presented by semiconductor amplifiers is represented by the relatively high intermodulation level, which is a hindrance to their use in wavelength division multiplexing communication systems. Fibre amplifiers utilise a length of an active optical fibre (or of an active integrated optical guide), i.e. a fibre or guide in which the core is doped with ions with spectroscopic emission bands in the regions of interest for telecommunications, in particular rare earth ions such as neodymium or praseodymium (for systems operating in the so-called second window, around 1.3 µm) and erbium (for the third window, around 1.55 µm). Amplification is obtained by launching into the core a pump radiation of suitable wavelength, different from that of the signal to be amplified. The main problem encountered when using these amplifiers is the need to employ dichroic couplers to send the signal to be amplified and the pump signal into the fibre or guide, and these couplers are difficult to integrate and are quite expensive. Furthermore, the amplifier switching on and off times, which depend on the fluorescence time of the reactants, are relatively long (of the order of some milliseconds for erbium). Recently, semiconductor amplifiers have also been proposed which have a low intermodulation level, so that they can be usefully employed with wavelength division multiplexing systems. An amplifier of this kind is described by G. Soulage et al. in the paper "Clamped gain travelling wave semiconductor optical amplifier as a large dynamic range optical gate", presented at the ECOC'94 Conference, Florence, Italy, 25-29 September 1994. The device is composed essentially of a distributed-feedback laser diode which, when brought to stimulated emission conditions, is capable of amplifying wavelengths in the third window. The device shows a gain that is constant for a wide range of input signal powers (at wavelengths other than Bragg wavelength) and does not depend on the input signal polarization. This device, in turn, has the drawback that the power emitted because of the laser effect is not exploited; moreover, like all semiconductor amplifiers, it is relatively noisy. According to the invention, instead, there is provided an optical amplifier that has high performance and low noise and that does not require dichroic coupling devices. EP-A-0 409 177 (see Fig.4 thereof) describes a gain clamped semiconductor laser situated at the output of an erbium doped fibre amplifier, whose lasing wavelength is used for pumping the fibre, the signal wavelength amplified by the fibre passing through the semiconductor laser as an output, with possible further amplification taking place in the semiconductor laser, depending on the signal wavelength. EP-A-0 525 412 describes a semiconductor laser situated between an undoped input fibre and a doped output fibre amplifier, the laser being used as a multiplexer to combine the input signal and the laser emission wavelength which is used to pump the doped fibre for signal amplification. The amplifier according to the invention comprises: an input stage comprising a first active optical guide amplifier; an output stage comprising a second active optical guide amplifier; and an intermediate stage, comprising a semiconductor amplifier which: is associated with means for producing a gain clamping; is capable of emitting a radiation having a different wavelength from that of an optical signal to be amplified and lying in a band corresponding to the pump band of the active optical guide amplifiers; is biased by a current bringing it to emission conditions; and is coupled to the guides forming the input and output stages in such a way as to send into them, as pump radiation, the radiation emitted through stimulated emission. Gain clamping may be obtained by means of a grating made within the semiconductor amplifier, which therefore is essentially a distributed-feedback laser, or by means of two optical fibre gratings, connected to opposite sides of the semiconductor amplifier and partially reflecting radiation at the semiconductor amplifier stimulated emission wavelength. The invention meets the requirements indicated above: the gain clamped amplifier is less expensive to be made than dichroic couplers and its use as the source of the pump radiation makes the structure both simple and compact. Moreover, the noise introduced by the semiconductor amplifier is added to a signal that has already been amplified by the input stage (which, being a fibre or integrated optical amplifier, is not very noisy in itself), and therefore its influence is lessened. Also, switch-off characteristics no longer depend on the reactant fluorescence time, but on the semiconductor amplifier, which is controlled by a current and thus has much shorter switch-off times (of the order of a nanosecond). Finally, the amplifier output power is essentially constant, since on the one hand the semiconductor amplifier gain is essentially constant for a wide range of input powers, and on the other the intensity of the input/output amplifier pump radiation (i.e. of the semiconductor amplifier emission) essentially varies in a way that is inversely proportional to the intensity of the radiation arriving at the semiconductor amplifier. For the sake of further clarification, reference is made to the enclosed drawings, where: Figure 1 is a schematic diagram of the amplifier according to the invention; Figure 2 is an outline of a gain clamped semiconductor amplifier; and Figure 3 is a schematic representation of a variant embodiment. In Figure 1, the amplifier according to the invention, indicated by dashed-line block 1, comprises an input stage 2 and an output stage 3, which in the illustrated example are each made up by a section of optical fibre doped with rare earth ions (in particular Er ions, if the signal to be amplified has a wavelength within the third window, or Nd or Pr ions if the signal to be amplified has a wavelength within the second window), and an intermediate stage 4 comprising a gain clamped semiconductor amplifier, to which the fibres making up amplifiers 2 and 3 are joined. Semiconductor amplifier 4 is biased by a current such that it is brought to laser emission conditions, so that it acts as a pump radiation source for amplifiers 2, 3. Amplifier 2 shall be pumped in reverse direction with respect to the signal to be amplified, while in amplifier 3 the signal and the pump radiation propagate in the same direction. The fibres that make up amplifiers 2, 3 are joined to amplifier 4 in a wholly conventional manner. By means of optional input and output isolators 5, 6, amplifier 1 is connected to two fibre sections 7, 8 which convey the signals to be amplified and respectively the amplified signals. Gain clamped semiconductor amplifier 4 may be for instance an amplifier like that described in the paper by G. Soulage et al, which is shown schematically in Figure 2. Amplifier 4 is essentially a distributed feedback semiconductor laser, in which the optical feedback is obtained by means of a grating 9. The terminal facets are equipped with anti-reflection coatings 10 to suppress Fabry-Perot resonances. The electrode and the conductor for connection to the bias current source are indicated by references 11, 12. Considering by way of example the case in which the signals to be amplified have a wavelength within the third window (1.55 µm), so that amplifiers 2, 3 are realized with erbium doped fibres, amplifier 4 will be designed to amplify signals whose wavelength is, for instance, between 1.53 µm and 1.56 µm and to emit a radiation within the pump band of erbium (1.48 µm). The skilled in the art has no difficulty in designing the grating in such a way as to obtain the desired values. With the set-up described above, the signal to be amplified (e.g. a wavelength division multiplex signal), undergoes three successive amplifications and therefore good performance is obtained. Values of the order of ten dB for the input and the semiconductor amplifiers, and of the order of 5 dB for the output amplifier are easily obtainable. Semiconductor amplifier 4, as stated above, has an essentially constant gain, which does not depend on input power. On the contrary, the power emitted due to laser action, and therefore the fibre amplifier pump power, decreases as the input power increases, thus causing the fibre amplifier gain to decrease. However, in the whole output power remains essentially constant. To obtain amplification values like those stated above for fibre amplifiers, fibre lengths of the order of about ten meters will be sufficient. Therefore the device, including the isolators, can be mounted in a module of reduced size (a few centimetres on each side). In the embodiment shown in Fig. 3, gain clamped semiconductor amplifier 4 comprises a conventional semiconductor amplifier 40 connected to a pair of optical fibre gratings 20, 30 capable of partially reflecting the wavelength (e.g. again 1.48 µm) emitted by amplifier 40, thereby realising a cavity effect. Reflection must be only partial, as part of the radiation emitted by amplifier 40 is to pump optical fibre amplifiers 2, 3. Gratings 20, 30 may be formed in the end portions of fibres 2, 3, as shown in the drawing, or may be formed into sections of conventional fibres which are then joined to fibres 2, 3. It is evident that what is described above is given by way of non-limiting example and that variations and modifications are possible without departing from the scope of the invention as defined by the claims. In particular, the input and output stages may be realized by means of integrated optical guides of the active type. These, too, can easily be coupled with a semiconductor amplifier.

Claims: Optical amplifier, characterized in that it comprises: an input stage (2) comprising a first active optical guide amplifier; an output stage (3), comprising a second active optical guide amplifier; and an intermediate stage (4; 20, 30, 40), comprising a semiconductor amplifier which is associated with means (9; 20, 30) for producing a gain clamping, is capable of emitting, by stimulated emission, a radiation at a wavelength other than that of an optical signal to be amplified and lying in a pump band of the active guide amplifiers, is biased by a current such that it is brought to stimulated emission conditions, and is coupled to the guides making up the input and output stages (2, 3) so as to send into them, as pump radiation, the radiation emitted by stimulated emission. Optical amplifier according to claim 1, characterized in that the means for realising a gain clamping comprise a grating (9) made within the semiconductor amplifier (4). Optical amplifier according to claim 1, characterized in that the means for realising a gain clamping comprise a pair of optical fibre gratings (20, 30) connected to the semiconductor amplifier (40) and arranged to partially reflect the radiation emitted by the semiconductor amplifier (40) by stimulated emission. Optical amplifier according to claim 3, characterized in that the optical fibre gratings (20, 30) are made within the end portions of active optical fibres forming the input and output stages (2, 3). Optical amplifier according to any preceding claim, characterized in that it also comprises an input isolator (5) and an output isolator (6) for connection of the input and output stages (2, 3) to optical transmission lines (7, 8) which respectively convey the signals to be amplified and the amplified signals.

Inventors: GAMBINI PIERO, GAMBINI, PIERO
Applicants: CSELT CENTRO STUDI LAB TELECOM, CSELT CENTRO STUDI E LABORATORI TELECOMUNICAZIONI S.P.A.
Application year: 1995
Main classification: H01S3
IPC codes: H01S3, G02F1, H01S5