NZ240957A

NZ240957A - Optical fibre amplifier

Info

Publication number: NZ240957A
Application number: NZ240957A
Authority: NZ
Inventors: Rolf Heidemann; Thomas Pfeiffer; Manfred Kaiser
Original assignee: Alcatel Australia
Priority date: 1990-12-24
Filing date: 1991-12-12
Publication date: 1994-04-27
Also published as: CA2074813A1; ES2074357T3; EP0516843B1; WO1992011561A1; EP0516843A1; AU8960191A; AU648365B2; DE59105276D1

Description

<div class="application article clearfix" id="description"> 240557 ^W2.'9D CCMipi itpjssitOi.Cczf/.ln D, U , » U'.- •„ ^PR t99^ P ; - wecm NEW ZEALAND PATENTS ACT 1953 COMPLETE SPECIFICATION "OPTICAL AMPLIFIER" N.Z. PATENT OFFICE 12 DEC 1901 WE, ALCATEL AUSTRALIA LIMITED, A Company of the State of New South Wales, of 280 Botany Road, Alexandria, New South Wales, 2015, Australia, hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: I This invention relates to an optical amplifier using a length of fibre doped with an active laser substance. Optical amplifiers serve to amplify light signals which arc transmitted over a transmission link, particularly over an optical fibre. The signal strength is reduced 5 by attenuation, so that the signals have to be amplified. Such an optical amplifier is known, for example, from "Electronics Letters", 19th July 1990, Vol. 26, No. 15, pp. 1102-1104. It is a fibre amplifier comprising an erbium-doped length of fibre. Erbium is an active laser substancc, which is excitcd by means of a semiconductor laser acting as a pump laser. The semiconductor laser 10 produces light signals of the wavelength X = 1480 nm, for example. The light signals are entered into the doped length of fibre through an optical coupler. Between the transmitting and receiving devices at the ends of the optical fibre and the optical amplifier in the optical fibre, optical isolators arc provided. This optical amplifier suffers from the drawback that the pump light generated 15 by the semiconductor laser is reflected back into the laser at all points in its propagation path where refractive index step differences occur (ic., in the region of the optical coupler or at the interface between the optical fibre and the erbium-doped length of fibre). This degrades the efficiency of the semiconductor laser; the reflected light may cause variations in the amplitude of the pump light emitted by the scmiconduc-20 tor laser bccausc the activc laser layer in the semiconductor laser combines with a part of the transmission link (ic., up to the point with the refractive index step difference) to act as a laser resonator. This results in a cliangc in the emission frequency of the semiconductor laser and in amplitude variations of the pump light. Especially if these variations lie in the low-frequcncy range, eg., in the kilohertz range, the am-25 plifying effect of the length of fibre is temporarily cancelled: The optical trasmission may be interrupted. 2 It is desirable to provide an optical amplifier which ensures interfcrcncc-frcc amplification of the light signals. It is also desirable to provide a method of operating an optical amplifier. This specification discloses an optical amplifier comprising a length of doped 5 fibre and an optical coupler, the length of doped fibre containing an active laser substance and being capable of amplifying light signals, and the optical coupler connecting the length of doped fibre at one end thereof to a first semiconductor laser which is supplied with a direct current used as a pump current, wherein the direct current used as the pump currcnt has an alternating current superimposed thereon 10 which is generable by a first alternating-current source. One advantage of the invention is that it eliminates the need for the costly installation of additional optical isolators between the optical coupler and the semiconductor laser directly in front of the latter to prevent feedback of the pump light into the semiconductor laser. 15 According to a particularly advantageous embodiment of the invention, the frequency of the alternating currcnt lies above the reciprocal lifetime of that excited energy level of the active laser substancc, eg., erbium, which brings about the amplification of the light signals to be transmitted. In this way, the variations in the amplitude of the pump light arc not "pcrccivcd" by the active laser substancc, ic., the 20 length of fibre acts like a low-pass filter for the varying components of the pump light signals. According to another advantageous embodiment of the invention, two semiconductor lasers arc provided the pump light of each of -which can be coupled into one of the ends of the length of fibre. 25 The invention will now be explained in greater detail with reference to the accompanying drawing, which shows one embodiment of the invention. 3 9 fj A 0 r; ▼ '' *V (i >' ;w J* As far as the invention relates to the method, the latter will be described together with the arrangement required to carry it out. The single figure shows an optieal amplifier. The optical amplifier forms part of a fibre-optic transmission link I; the latter 5 includes a length of fibre 2 doped with an active laser substancc, eg. erbium. On the fibre-optic link 1, light signals arc transmitted, which arc amplified in the length of fibre 2. The light signals have a wavelength of X = 1530-1570 nrn, for example. At a first end, the length of fibre 2 is connected to a semiconductor laser 4 via an optical coupler 3. The semiconductor laser 4 is, for example, an InGaAsP/InP, 10 InGaAs/AIGaAs or GaAlAs/GaAs laser which emits light from the wavelength range X = 750-870 nm, X = 960-1000 nm or X = 1460-1500 nm, respectively. It is connected to a DC source 5 via an electric conncction 42. Via an clcctric connection 42, it is connected to an AC source 6. Similarly, the length of fibre 2 is conncctcd at a second end to a second semi-15 conductor laser 8 via an optical couplcr 7. The second semiconductor laser 8 has two electric connections 81 and 82, via which it is conncctcd to the DC source 5 and the AC source 6, respectively. The DC source 5 supplies to each of the semiconductor lasers 4, 8 a direct currcnt whose value normally lies above the value of the laser threshold current. According to the invention, the value of the dircct currcnt lies bc-20 low that of the laser threshold currcnt and the latter is periodically exceeded by superimposing an alternating currcnt on the dircct currcnt. The AC source 6 generates the alternating currcnt which is superimposed on the direct current to form the pump currcnt. Bccausc of-the alternating current, the semiconductor lasers 4, 8 emit multimode pump light; for instance, light from a 25 semiconductor laser with an emission wavelength of X = 1480 nm, which has only few modes during direct-current operation, is split up by the alternating current 4 2 4 0 Q n> 7 component into 20-30 modes with a spacing of 80GHz. The frequency of the alternating currcnt preferably lies above the reciprocal lifetime of the cxcitcd energy level of the erbium in the length of fibre 2. Since the lifetime of the energy level lies in the millisecond range, the alternating current must have a frequency in the lower 5 megahertz range. Then, the light signals to be transmitted and amplified will not be modulated at the frequency of the pump light. The AC source 6 generates a sinusoidal or square-wave alternating current, for example. Instead of the single AC source 6. a separate AC source may be provided for each semiconductor laser 4, 8. Advantageously, a phase shifter for producing a phase 10 difference between the alternating currents supplied to the semiconductor lasers 4, 8 is inserted between one of the two AC sources and the associated semiconductor laser 4 or 8. This also reduccs an interference from one semiconductor laser to another caused by the pump light of the respective other laser. The phase shifter may be present alone or in conjunction with a switch through 15 which the alternating currcnt and the dircct currcnt can be applied to the semiconductor lasers 4, 8 by turns. This prevents any interference from one semiconductor laser to the other. The switch is a D flip-flop with two complementary outputs, and the alternating currcnt a square-wave signal, for example. The switch may also connect the AC source 6 to the semiconductor lasers 4, 8 without a phase shifter being 20 present. In another embodiment, only a single couplcr and a single semiconductor laser 4 are present. 3ctwccn the AC source 6 or AC sources or the DC sourcc 5 or DC sources, on the one hand, and the elcctric connections 41, 42, 81, 82, on the other hand, electric 25 amplifiers may be provided. Ahead of and bching the optical amplifier, optical isolators 9, 10 arc preferably provided. 5 </div>

Claims

<div class="application article clearfix printTableText" id="claims"> V 0 9 5 What we claim is:

1. An optical amplifier comprising a length of doped fibre and an optical coupler, the length of doped fibre containing an active laser substance and being capable of amplifying light signals, and the optical coupler connecting the length 5 of doped fibre at one end thereof to a first semiconductor laser which is supplied with a direct current used as a pump current, wherein the direct current used as the pump current has an alternating current superimposed thereon which is generated by a first alternating-current source.

2. An optical amplifier as claimed in claim 1, wherein the frequency of the 10 alternating current lies above the reciprocal lifetime of an energy level of the active laser substance which amplifies the light signals during its decay.

3. An optical amplifier as claimed in claim 1 or claim 2, wherein at the other end of the length of fibre, an additional optical coupler is provided which connects the length of doped fibre to a second semiconductor laser which is 15 supplied with a second pump current composed of a direct current and an alternating current.

4. An optical amplifier as claimed in claim 3, wherein the alternating current contained in the second pump current is generated by a second alternating-current source. 20

5. An optical amplifier as claimed in claim 3, wherein the alternating current contained in the second pump current is generated by the first alternating-current source.

6. An optical amplifier as claimed in claim 5, wherein the alternating currents J," " " | $ (rj ,1 L\ FEB 1934 ! I j 2' 0 9 contained in the pump currents is supplied alternately to the first semiconductor laser and the second semiconductor laser via a switch.

7. An optical amplifier as claimed in claim 6, wherein the switch includes a D flip-flop. 5

8. An optical amplifier as claimed in any one of claims 3 to 7, wherein a phase shifter is provided for changing the phase difference between the alternating currents contained in the two pump currents.

9. An optical amplifier substantially as herein described with reference to the accompanying drawing. 10

10. A method of operating an optical amplifier wherein light signals are amplified by an active laser substance contained in a length of fibre, with pump light for exciting the active laser substance being generated by a semiconductor laser which is supplied with a direct current used as a pump current, wherein an alternating current is superimposed on the direct current used as the pump 15 current.

11. A method as claimed in claim 9, wherein the alternating current contained in the pump current has a frequency lying above the reciprocal lifetime of an energy level of the active laser substance which amplifies the light signal during its decay. 14 FED 1934 , teC-BVlHD h 24 0 9 5

12. A method of amplifying optical signals using an optical fibre doped with an active laser material, the method being substantially as herein described with reference to the accompanying drawing. ALCATEL AUSTRALIA LIMITED P.M. Conrick Authorized Agent P5/1/1705 p— </div>