JPH06500896A - optical amplifier - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed description of the invention] optical amplifier Optical amplifiers are transmitted over transmission lines, especially optical fiber waveguides (optical fibers), To amplify an optical signal that must be amplified because it is weakened by attenuation. functions.

An optical amplifier having the characteristics according to the preamble of claim 1 is described, for example, by M. Yoshida et al. Sentence “Deyelopmenl of Compact Er” -doped Fiber Amplifiers for Prac AppliC sth ons″.

Technical Digest on Open Cal Amplifi! r s and their Applications 1990. (Opti cal So, ciejy of America, Washington.

DC1990), Volume13. Known from pages 282 to 285 Ru.

It is a fiber amplifier with erbium doped fiber. Erbiu The laser active material is excited by a semiconductor laser that acts as a pump laser. It is quality. In this example, the semiconductor laser emits light at a wavelength of λ-1480m. Ru.

This light is introduced into the doped fiber by an optical coupler. DC current is half Used as operating current to control conductor laser.

Instead of a single pump laser as shown in Figure 1a of this document, the pump laser are coupled to each end of the optical fiber, as shown in Figure IC. A user can also be provided.

The disadvantage of this optical amplifier is that the pump light generated by the semiconductor laser has a refractive index shift. (e.g. in the area of the optical coupler or in the area of optical fibers and erbium-doped in the semiconductor laser at all points along the propagation path (at the transition between the fiber and the fiber). It is reflected back to the This degrades the efficiency of the semiconductor laser. reflected The laser active layer in the semiconductor laser The semiconductor laser operates as a laser resonator with the This causes amplitude fluctuations in the emitted pump light.

This changes the emission frequency of the semiconductor laser on the one hand and the amplitude of the pump light on the other hand. give movement. In particular, the amplification effect of fibers is such that vibrations can be reduced to low frequency ranges, e.g. kilohertz. It is temporarily erased when it occurs in this range. This blocks optical transmission under certain circumstances. do.

If there are two pump lasers (semiconductor lasers in each example), the aforementioned problem exists for each of the two pump lasers. Additionally, in that case each pump laser There is a problem with the remaining pump light entering the pump laser on the other side, which is This causes instability, leading to the problem of vibration during amplification mentioned above.

Therefore, optical isolators are currently used in front of the pump laser.

The object of the present invention is to provide one pump laser or two pump lasers, The objective is to produce an optical amplifier that guarantees interference-free amplification.

This object is met as indicated in claim 1.

An advantage of the present invention is that the pump light is prevented from being reflected back into the semiconductor laser. For this purpose, an expensive optical isolator between the optical coupler and the semiconductor laser just before the latter is required. The aim is to eliminate the need for installation of

The operating current or frequency modulated by the laser active material provides amplification of the transmitted optical signal. , above the reciprocating lifetime of the excited energy level of erbium, for example. . In this way, oscillations in the pump light amplitude are "perceived" by the laser-active material. In other words, the fiber has a low-pass Acts as a filter.

Further advantageous constructions are described in claims 2 to 5.

FIG. 1 shows an optical amplifier according to the invention with a single pump laser.

FIG. 2 shows an amplifier according to the invention with two pump lasers.

In FIG. 1, an optical amplifier is inserted into an optical waveguide transmission line 1.

It has a fiber 2 and is doped with a laser active material, e.g. erbium. ing. Optical signals are transmitted through optical waveguide transmission line 1 and amplified in fiber 2. be done. The optical signal has a wavelength of λ=1530 to 1570 nm. fiber 2 One end 21 of is connected to the semiconductor laser 4 by the optical coupler 3 . semiconductor laser 4 is, for example, InGaAsP/InP, InGaAs/AlGaAs or G a　AI　　A　s　/　G　a　As　s laser, which has a wavelength of λ=750 to 870nm, λ-960 to 11000n or λ=1460 to 1500n It emits at wavelengths in the range m. Electrical contacts 41 connect it to the DC power source 5. child The description of the optical amplifier in FIG. 1 so far corresponds to known types.

According to the invention, the operating current of the laser is not a DC current, but a modulated current.

For this purpose, the semiconductor laser 4 of the structure shown in FIG. It has a feeler 42.

The AC power source 6 generates, for example, a sinusoidal or square wave alternating current.

The AC power source 6 generates alternating current, which is coupled to the semiconductor power source along with the direct current from the DC power source. Forms the operating current of the laser 4 (pump laser). In other words, the operating current is The current is a superimposed direct current, or the operating current is (periodically) modulated. This is the direct current that is generated.

Instead of a DC power supply 5 or an AC power supply 6 structure, a modulated or C power supply also has an operating voltage. can be used as a source. If only the operating current is modulated in a certain way No.

The semiconductor laser 4 is a multimode pump based on an alternating current or modulated operating current. radiate the target. For example, light from a semiconductor laser is a minority mode caused by direct current. It emits at λ = 1480 nm and has a mode spacing of 80 GHz. It is divided into 20-30 modes by the alternating current part.

The frequency of the AC, ie the modulation frequency of the operating current, is the excitation of the erbium in the fiber 2. above the reciprocating lifetime of the generated laser activation energy level. energy Since the lifespan of the megahertz level is in the millisecond range, the AC operates at frequencies in the low megahertz range. Must be equipped. This is where the pump light frequency is transmitted and amplified modulating the desired optical signal.

As mentioned in the introduction, two pump lasers are provided on each side of the amplifying fiber. There are optical fiber amplifiers. The problem of reflection of pump light back into the pump laser is eliminated. By operating the pump laser with an operating current, which is a modulated direct current, can be solved as originally shown.

FIG. 2 shows a structural example of such a solution. The left part of the drawing corresponds exactly to Figure 1. , so this part does not need to be explained again. second end 22 of the fiber is connected to a second semiconductor laser 8 by an optical coupler 7. This laser has two electrical contacts 81 and 82 connecting it to the source 5 or AC power source 6 Ru.

Instead of a single AC power supply 6, each semiconductor laser 4 and 8 has its own AC power supply. It is also possible to have The same also applies to the DC power supply 5.

of the type shown in FIG. 2, or as described above for the example of a single pump laser. Operating power supply by other means such as Let it be adjusted.

However, the reciprocating interference effects of the pump laser cannot be removed. So This is why optical isolators are conventionally placed immediately in front of the pump laser. Book The invention consists of two pump lasers that are alternately switched on and off so that only one is active. allows the isolator to be removed if one is active and the other is inert . This prevents reciprocating interference between the semiconductor lasers 4 and 8. Represents the modulation of the operating current of the laser. The multimodality of lasers is due to the switching frequency is automatically guaranteed by switching if is high enough.

If it is high enough due to the multimodal nature of the pump laser, it is actually higher than the reciprocating lifetime of the excited energy level of the laser-active material. This is required in all examples as explained at the beginning.

Average pumping in fiber 2 by two alternately switched on pump lasers The laser power is only as large as that of a single permanently operating pump laser, but Pumping in two directions simultaneously has the advantages of low noise coefficient and high power Achieve both. Additionally, each laser is only used half the time, which makes it less reliable. have a positive impact on

For example, to supply both pump lasers with an operating current that is switched on alternately. An HF generator with two inverting outputs can be used, one of which carries the current conducts current, and the other conducts no current.

To switch the pump lasers on and off, turn on the operating voltage of one pump laser. If the flow was completely switched off during its idle phase or if the pump laser It does not matter whether it is only lowered below the threshold current.

Switching the two pump lasers on and off alternately can also be done through a switch. by means of a device that alternately supplies square wave AC to one and the other pump laser. achieved.

Regardless of how it is generated, the pump laser operating current turns on and off It may be a square shaped D current C that is switched. However, on and It is a square wave DC that is switched off and on, and the AC current is permanently or continuously switched off. is superimposed only during the on-phase of the laser, so that the modulated DC The operating current through the pump laser during phase -c flows. This is the result of two laser The switching frequency alternately switches on each pump laser during the on phase. This is especially necessary if the In that case, DC The modulation provides multimodality during the on phase.

international search report international search report PC Ding/EP 91102458 S^　54552 INIPO1n+Oka111 Continuation of front page (72) Inventor: Kaiser, Manfred, Federal Republic of Germany, B-7254 Mingen, Zeppelinstrasse 2

Claims (5)

[Claims] (1) at least one serving as a pump light source and energized by an operating current; a fiber containing a laser-active material that amplifies an optical signal coupled to a semiconductor laser In an optical amplifier with The modulation frequency of the operating current increases the optical signal when it decreases. be above the reciprocating lifetime of the energy level of the laser-active material. Characteristic optical amplifier. (2) The operating power source includes DC power source and AC power source, and the output current forms the operating current. The optical amplifier according to claim 1. (3) Two semiconductor lasers each connected to one end of the fiber as a pump light source. The operating power source is two semiconductor lasers that are alternately switched on and off. 2. The method according to claim 1, wherein current is supplied to the two semiconductor lasers so as to optical amplifier. (4) The operating current for each of the two alternately switched on semiconductor lasers is DC switched on and off at a defined frequency and modulated by it 4. The optical amplifier according to claim 3, wherein the optical amplifier is a current. (5) The operating current for each of the two alternately switched on semiconductor lasers is constant. A charge that is a DC current that is switched on and off modulated at a set frequency. The optical amplifier according to item 3.