DE4016331A1 - Brillouin optical fibre amplifier for optical heterodyne receiver - uses feedback of light from pumped laser eliminating need for second local laser - Google Patents

Abstract

The Brillonin optical fibre amplifier feeds the light from a pumped laser (LD) to the transmission path via a coupler (K1). The free arm of the latter allows part of the light output of the pumped laser to be fed back for acting as a local laser wave superimposed on the incoming optical signal. A mirror (S) is coupled to the free arm of the coupler (K1) at one end, its opposite end coupled to the photodiode (PD1) for the incoming optical signals. ADVANTAGE - Requires single pumped laser source.

Description

The invention relates to an optical cover tion receiver according to the preamble of claim 1.

The Brillouin effect describes the scattering optically Waves of acoustic molecular vibrations. With less The intensity of the optical wave is only the spontaneous one Brillouin scatter present due to the wave propagation has no noticeable influence in the glass fiber. Will the Intensity increases, stimulated additionally occurs Brillouin scatter and excite a new optical wave, the Stokes wave, whose direction of propagation is opposite to that of the stimulating wave and its Frequency is lower than the Stokes frequency. A Single-mode fiber now also leads with a reflection-free path and one-sided excitation two opposite waves. The injected wave that carries the useful signal and that moved away from the transmitter, and an induced interference wave that deprives power of the useful wave and that to the transmitter runs back. Because of the exponential relationship between stimulated Brilloni scatter and intensity The stimulating wave takes off with a further increase in intensity the power induced in the Stokes wave  disproportionately too, so that from the useful wave transferable power is a typical fiber limit is approaching. Any further increase in transmission power is amplified only the Stokes wave, which directly returns the power transported back to the transmitter. The fiber now acts as Mirror that additionally reflects the reflected beam the frequency shifts. This process, the transformation all "pump photons" in "Stokesphotons" is also called "Pump depletion" referred to.

Characteristic variables of the Stokes wave are the natural line width Δf _{B of} the stimulated Brillouin scattering and the Stokes shift, the frequency distance Δf _s by which the frequency of the Stokes wave is lower compared to the frequency of the exciting wave.

For optical messaging, and here particularly in the case of coherent transmission methods, the stimulated Brillouin scattering because of the limiting Effect on the transmission power and the generation of Disruptive agents a very harmful effect. On the other hand enables the targeted application of the stimulated Brillouin scattering in the Brillouin fiber amplifier direct amplification of optical signal waves.  

The Brillouin fiber amplifier consists of a glass fiber that is connected at one end to a pump laser via a coupler. The optical signal wave to be amplified is fed into the fiber at the opposite end and is increasingly coupled out again at the free output of the coupler ( FIG. 1). The reinforcing medium is the glass fiber, which is also the transmission link.

The pump wave of frequency f _P is coupled into the glass fiber at the far end and runs through the fiber counter to the direction of signal transmission. If the pump power is sufficiently high, the Stokes wave with the frequency f _s = f _p -Δf _{s is} generated during the passage through the fiber, which returns to the fiber end in this configuration. The signal wave of frequency f _{SIG which} is fed in simultaneously at the beginning of the fiber is then amplified by the Stokes wave if both have the same frequency. The pump frequency must therefore always be controlled so that it is always exactly Δf _s above the signal frequency. The gain bandwidth in this case is equal to the natural line width of the Brillouin scatter and is in the order of 40 MHz for single-mode fibers. The gain factor of the Brillouin fiber amplifier depends on the Brillouin gain, the effective core diameter of the fiber, the effective fiber length and the pump power available in the fiber.

Although the principle of the Brillouin fiber amplifier is This component has been known for a long time because of the relatively low transmission bandwidth so far observed and only a few applications are known.

The classic application for a Brillouin Fiber amplifier is the selective amplification of a modulated optical carrier before photodetection. The amplified carrier signal enables shot noise limited homodyne reception even without the otherwise required optical phase locked loop. It will be with a pump power of 5 mW to reinforce the carrier 48 dB achieved with a fiber length of 30 km.

According to "N.A. OLLSON, J. P VAN DER ZIEL: Characteristics of a semiconductor Laser Pumped Brillouin Amplifier with Electronically controlled bandwidth, J. Lightwave  Technology LT-5 (1987), pages 147-153 "it is possible that natural gain range of Brillouin Fiber amplifier by frequency modulation Pump laser shaft to enlarge significantly.

At the cost of a gain factor reduced by 10 dB became the natural Brillouin reinforcement range increased by a factor of 10. In a transmission experiment was a transmission signal with a bit rate of 90 Mbit / s Transmission laser (line width = 10 kHz) externally modulated and transmitted over a 30 km long fiber optic link. in the The fiber was used with an ordinary receiver optical coupler completed. During a coupler branch was connected to the direct recipient via the other branch the pumping power into the fiber fed. The pump laser (line width = 10 kHz) gave one optical power of 2.9 mW into the fiber and was directly frequency modulated. The one with this configuration possible gain of 9 dB improved the Receiver sensitivity around 8.5 dB.

The use of the Brillouin fiber amplifier for selective channel amplification and demodulation in one  optical frequency division multiplex system was described in "R. W. TKACH et. al .: Performance of a WDM Network Based on Stimulated Brillouin Scattering, IEEE Photonics Techn. Lett., Vol. 1, May 1989, pages 111-113 "and experimentally examined. As stated above, were a direct recipient and a pump laser (line width = 10 MHz, pump power = 12 mW) via an optical coupler with the end of the fiber optic link connected at the receiving end. The Pump laser was with a sawtooth signal (frequency = 10 MHz) directly modulated and thereby caused a approximately rectangular Brillouin gain curve with approx. 600 MHz bandwidth. The gain factor was 30 dB. The channel was selected by changing the frequency of the Pumplasers so that the channel, its center frequency was within the amplifier range, selectively amplified, and due to the chosen modulation scheme, too was demodulated at the same time. At bit rates of 150 Mbit / s was a min. Channel spacing of 1.5 GHz required. The Sensitivity was increased by 8 dB.

The application of a Brillouin fiber amplifier in Connection with a heterodyne receiver was in "M. TSUBOKAWA, Y. SASAKI: Coherent FSK Transmission  Experiment using Brillouin Amplification in a Single-Mode Fiber, J. Opt. Commun. 10 (1989), pages 42-47 " examined. An FSK modulated laser signal (bit rate = 30 Mbit / s Line width of the transmit laser = 9 MHz) was over a 52 km long fiber optic line on one Given heterodyne receiver. The free entrance of the for Summary of reception and local laser signal necessary optical coupler was used for the feed the pump shaft used. The pump laser (line width = 10 MHz, Pump power = 2.5 mW) was with a 1 MHz signal directly modulated to an amplifier bandwidth of 180 MHz to achieve. The reception signal was received by the Brillouin Fiber amplifier amplified by 4 dB. The rise in Gain factor was 1.6 dB per mW pump power.

The object underlying the invention, a Brillouin fiber amplifiers to expand so that this Component additionally as an optical overlay receiver selective optical channel amplification can be used can, is characterized by the in the main claim Invention solved.

Further embodiments of the invention are in the Subclaims marked.  

The advantages that can be achieved with the invention exist especially in saving a separate one Local laser.

In contrast to that of "M. TSUBOKAWA, Y. SASAKI: Coherent FSK Transmission Experiment using Brillouin Amplification in a Single-Mode Fiber, J. Opt. Commun. 10th (1989), pages 42-47 "proposed solution is only here a laser is needed in the receiver. In addition to the Reducing costs also simplifying Frequency control since there are no longer two lasers on one third (the transmitter laser) must be tuned. The location the IF at about 11 GHz, which is due to the Stokes shift yes is fixed, is also particularly cheap because for this frequency range because of the intensive use for the Satellite radio already has a variety of broadband and low-noise electronic components are present.

Except on the reception side, the relative location of Transmitter and pump lasers also regulated to each other on the transmission side using a broadband photodetector (PD) Pump and transmission frequency on the transmission side detected and can be used to derive the control signal.  

With a given signal-to-noise ratio, the Field length can be increased.

The electronically adjustable bandwidth of the Brillouin Fiber amplifier enables adaptation of the amplifier to the broadcast signal. This has the effect of an optical one Prefilter and thus reduces the demands on the electrical filtering.

Compared to a direct reception system, here are still coming the basic advantages of heterodyne technology, e.g. B. Sensitivity gain up to 20 dB depending on Transmission process added.

The disadvantage of the basic version is that of Transmission medium predetermined relatively narrow Gain bandwidth of the Brillouin fiber amplifier, the is limiting for the possible transmission rate. The Direct modulation of the pump laser to increase the bandwidth is not recommended for this receiver concept because the spectral broadening the use of the pump laser as  Local lasers in a heterodyne system as good as excludes. Another option is to use it an external phase modulator. About bit rate and coding can shape and bandwidth of the gain curve can be set. Well suited for this is e.g. B. the Modulation with a digital pseudo noise sequence and NRZ coding. A disadvantage compared to direct modulation are the higher additional insertion loss due to the Modulator as well as the greater outlay due to the additional required components coupler and phase modulator.

An embodiment of the invention is in the drawing shown and is described in more detail below.

Show it

Fig. 2 basic version of the extension for optically heterodyne receiver,

Fig. 3 shows a first variant of the illustrated in FIG. Superposition optical receiver 2,

Fig. 4 optical heterodyne receiver with control of the IF frequency at the transmitting side,

Fig. 5 shows a second variant of the optical superimposition receiver shown in Fig. 2.

The analysis of an optical transmission system, consisting of consisting of transmitter, fiber transmission link and straight-ahead receiver with an upstream Brillouin fiber amplifier shows that this system already all for the overlay contains the necessary components.

The power of the optical pump shaft is over one optical coupler the share for the local laser power branched off. The intermediate frequency is not in this case more variable, but through the Stokes shift and is around 11 GHz.

The Brillouin fiber amplifier becomes a superimposed receiver for optical signals if the portion of the pump power at the output of the free arm of the feed coupler K 1 is reflected with the aid of a mirror. The reflected wave is superimposed on the light coming from the transmitter, which contains the information, on the photodetector PD 1 . The feed coupler thus also takes over the function of the superimposition coupler. Instead of a coupler with a mirrored output, a second coupler can also be used. The polarization adjuster P is additionally required in order to adapt the polarization of the reflected wave to the polarization of the received wave.

An additional polarization controller P in the reflection branch is also necessary to the polarization of the reflected wave to the polarization of the received Adapt signal wave.

Fig. 2 shows the basic version of the extension of a Brillouin fiber amplifier to an optical heterodyne receiver according to the invention.

The light wave emanating from the pump laser is fed into the transmission path via the coupler K 1 and propagates in the direction of the transmitter. The frequency of the pump wave f _P is chosen so that the frequencies of the signal wave f _SIG and the reflected Stokes wave f _S coincide (f _S = f _SIG = f _P -Δf _S ). The signal wave f _SIG is amplified by the Stokes wave during the parallel propagation of the two waves and then conducted at the coupler K 1 to the photodetector PD 1 of the receiver. The isolator I prevents Stokes and signal waves from causing repercussions on the laser. The branched-off at the coupler K 1 local laser portion of the pump wave is reflected by the mirror S at the output of the coupler K 1 and, after again, Kopplerdurchlauf also on the photodetector PD. The polarization controller P is used to adapt the polarization of the reflected wave to the polarization of the signal wave.

The photodetector PD generates from the overlay two optical waves one to be absorbed Light output proportional electric photocurrent, the u. a. at an intermediate frequency of about 11 GHz Spectral components of the mixed product from amplified signal and contains local laser power. This term, which also an optical one thanks to the Brillouin gain curve Has undergone preselection, will then go to the electrical demodulation processor.  

This processor also needs the control signal for the Deliver frequency stabilization of the pump laser, as well as at OFDM systems also a control signal for the Provide the frequency setting of the pump laser. in the the latter case becomes the frequency for the channel setting the pump laser set so that it by the amount of Stokes shift over the frequency of the one to be selected Channel lies.

As shown in FIG. 3, a second coupler K 2 can also be used instead of the mirror S.

Fig. 4 shows an optical heterodyne receiver according to the invention, in which, the regulation of the intermediate frequency on the transmission side. For this purpose, power is routed to a second photodetector PD 2 on the transmission side via an optical coupler. At the intermediate frequency of approx. 11 GHz, the photocurrent contains the mixed product of the pump and transmit signal, which is evaluated for the generation of the control signal, as in the case of the receiver control.

By inserting an additional external phase modulator EM and a third coupler K 3 , a heterodyne receiver with electronic bandwidth control as shown in FIG. 5 can be realized. This extension enables operation even at data transfer rates whose spectral bandwidth is larger than the natural Brillouin bandwidth.

Claims (5)

1. Extension of a Brillouin fiber amplifier to the optical superposition receiver with selective optical preamplification when using only a local laser in the receiver, characterized in that a portion of the power of the pump laser wave present at the output of the free arm of a first coupler (K 1 ) is fed back and the incoming signal wave is superimposed as a local laser wave in front of the photodiode (PD 1 ). 2. Extension of a Brillouin fiber amplifier according to claim 1, characterized in that for returning part of the existing pump power, the end of the free arm of the feed coupler (K 1 ) is completed with a mirror (S). 3. Extension of a Brillouin fiber amplifier according to claim 1, characterized in that, for recycling a portion of the available pump power of the free arm of the Einspeisekopplers is performed (K 1) in a second coupler (K 2). 4. Extension of a Brillouin fiber amplifier after Claim 1, characterized in that the scheme the transmission frequency of the pump laser (LD) on the transmission side he follows. 5. Extension of a Brillouin fiber amplifier after Claim 1, characterized in that by a additional external phase modulator and one optical coupler the bandwidth control done electronically.