GB2144598A - Laser telecommunications system - Google Patents

Abstract

The output of a spectrum stabilised transmitting laser (5, 15) is angle modulated by a signal to be transmitted and is coupled to a monomode optical fibre link (8). A second spectrum stabilised laser (10, 15) is located at the other end of the link. Each laser is arranged so that it can be injection locked by a signal received from the other so that it can oscillate in synchronism with the other laser. This arrangement provides a bi- directional link. Also disclosed is a demodulator for demodulating angle modulated optical signals. <IMAGE>

Description

SPECIFICATION Laser telecommunications system The present invention relates to laser telecommunications systems. The system to be described is particularly, but not exclusively, suitable for use in submarine applications and also for bi-directional data links.

The invention includes a laser telecommunications system comprising a spectrum stabilised first laser having an output which may be angle modulated by a signal to be transmitted and which is coupled to one end of a monomode optical fibre section, and a second laser coupled to the other end of said fibre section and adapted to be injection locked by a received signal and thereby oscillate in synchronism with said transmitting laser.

The second laser can act as a repeater and its output can be coupled to a further monomode optical fibre section. Such a repeater is of extremely simple design and may be produced in a relatively compact package. As the repeater laser is operated as a synchronised oscillator not as a laser amplifier the repeater does not amplify intensity varing noise thus reducing the error rate in digital transmissions and the distortion of analogue signals. By utilising a wavelength, preferably substantially 1.55 lim, at which the loss of the monomode fibre is at a minimum repeater spacing may be further increased.

In the present invention the second laser is spectrum stabilised and has an ouput which may be angle modulated by a signal to be transmitted, the system further comprising means for selecting which laser is to transmit and which to receive. In this way a simple bidirectional link may be set up which can transmit data in one direction at a time between two stations.

The signal at the receiver requires demodulation. This can be carried out electrically after heterodyne detection but such detection requires the use of a precisely tuned local oscillator. Also disclosed is a simple demodulation technique in which an angle modulated optical signal is divided into two, the two signals are passed along paths of different length, the signals are combined and the combined signal applied to a non-linear photodetector.

The invention will be described now by way of example only with reference to the accompanying drawings in which: Figure 1 is a diagrammatic representation of a laser telecommunications system; Figure 2 is a diagrammatic representation of a bi-directional data link, and Figures 3A, B and C illustrate a demodulation system.

The system of Fig. 1 comprises a transmitter 1 and a receiver 2 with a monomode optical fibre transmission line 3 therebetween.

One or more repeaters 4 may be spaced at intervals along the line 3.

The transmitter 1 comprises a spectrum stabilised first laser 5 which is provided with wavelength control means 6 arranged such that the laser 5 produces only a single mode output with a relatively narrow line width.

This spectrum stabilisation may be achieved by a wavelength sensing system as described in the Post Office's UK Patent Specification No. 2 007 01 5A entitled "Control Apparatus", or by means of an injection locking system as described in UK Patent Application No. 8104828 entitled "Spectrum Stabilised Laser Transmitter" and a paper of the same title by D.J. Malyon, D.W. Smith and R.W.

Berry published in the proceedings of the Third International Conference on Integrated Optics and Optical Fibre Communication 1981. The laser 5 may be a semiconductor laser for example a double heterojunction laser based on an InP substrate and having an InP Ga As active region. In submarine applications it may be desirable to provide a more complex laser at the transmitter terminal, which laser is capable of producing an extremely narrow line width and higher power into the transmission line, thus enabling a higher data rate and longer repeater spacing to be achieved. For this purpose a Neodymium/YAG laser may be used.

The laser 5 is angle modulated to produce an output signal which has analogue frequency modulation, or digital frequency shift keying or digital differential phase shift keying.

In order to angle modulate the transmitter laser 5 it is possible either to provide a separate modulator unit or, in the case of a semiconductor laser, to apply the signal to be transmitted as a perturbation to the bias current of the laser. This latter method is illustrated in Fig. 1 where the data input is along line 7 and directly frequency modulates the laser 5. If phase modulation is required a differentiator may be placed in line 7.

Recent work by Kobayashi et al published in Electronics Letters Vol 1 7 No. 10 has shown that almost frequency independent frequency deviation can be produced in an Al GaAs semiconductor laser having a centre wavelength of 834 nm for modulation fre quencies in the range 1OMHt GHz. It will be appreciated that the necessary laser bias current may be experimentally determined for the operation of an appropriate semiconductor laser in this manner about a wavelength of 1.55um. The modulation frequencies should be sufficiently high so that any wavelength sensing mode control of the laser is not influenced by the small and relatively rapid modulation induced wavelength changes. It is envisaged that for maximum deviation of the bias current by a few milliamps a frequency varia tion of about 1 GHz can be achieved.

Where other types of laser are utilised external modulator units are required, for example a phase modulator having a Lithium Niobate electro-optic device may be used to phase modulate a 1-523,um helium-neon laser.

The single mode angle modulated output from the transmitter laser 5 is coupled into a monomode optical fibre section 8 of the transmission line 3. The fibre is preferably manufactured to have minimum loss at an operating wavelength of 1 .55m thus taking advantage of the minimum possible theoretical loss region. Such a fibre has been described by Miya et al in Electronics Letters Vol 1 5 No. 4 under the title "Ultimate Low-Loss Single-Mode Fibre at 1.5ELm".

Preferably the optical fibre used is polarisation holding so that the transmitted single mode remains in the polarisation emitted by laser 5 and does not spread into the opposite polarisation during transmission.

Repeaters 4 are spaced along the transmission line 3 and connected to each other or to the transmitter 1 or receiver 2 as appropriate by sections of monomode optical fibre 8, 9 which it is envisaged may be of 60km or more in length. Each repeater 4 comprises a second laser 10 which is provided with wavelength control means 11 which monitors a part of the output of laser 10 by means not shown and operates to ensure that a cavity mode of the laser 10 is aligned with that of laser 5. In this way the laser light from the optical fibre section 8 which is incident on laser 10 injection locks the second laser 10 into the same single mode as the transmitting laser and causes it to operate in synchronism with the laser 5 thus reproducing the transmitted angle modulated signal.The second laser 10 is provided with a bias current above the threshold for oscillation and therefore operates as a synchronised oscillator and not as a laser amplifier. Operation as a synchronised oscillator reduces the effects of intensity fluctuations.

Alders formula:

where tf = full lock in bandwidth fo = centre optical frequency Q = cold cavity Q P1 = Injected signal power P2 = Power within cavity of locked laser which has been shown to apply to laser injection locking illustrates how the repeater spacing may be limited by the locking ability of the repeater lasers. If small tf, are utilised as may be feasible for transmitting digital signals by frequency or phase shift keying the signal may be permitted to attenuate to a lower power level (P,) before repeating.

Whereas if a high bandwidth is required as for transmitting an analogue signal the repeater spacing must be reduced in order that the repeater lasers 10 will lock in over the full bandwidth. Allowing 2 X 4dB coupling losses, 0.3dB/km fibre, 0.2dB/km splicing loss a repeater section length of 60km or more may be possible for 1 GHz lock-in range.

Preferably the repeater lasers 10 are semiconductor lasers thus allowing the repeater structure to be reduced to the minimum size.

The size of the repeater package is particularly important in submarine cables so that the repeaters do not unduly disturb the cable laying and burying operations. The simplicity of the repeater structure also increases overall reliability in a submarine cable as there are fewer components to fail.

An isolator 1 2 is interposed between the end of the fibre section 8 and the input to repeater laser 10 in order to prevent output from the back mirror of laser 10 travelling back along the transmission line 3 and interacting with the transmitted signal. Preferably the isolator is a magneto-optic Faraday effect isolator. If the lasers are ring lasers an isolator may not be required.

Clearly any number of repeaters 4 may be provided with intermediate monomode optical fibre sections depending on the required length of the overall line 3. The first repeater laser will be locked to the transmitting laser 5 and thereafter each repeater laser will lock to the last preceding repeater laser in the line 3.

The last optical fibre section 9 links the last repeater with the receiver 2. The receiver 2 comprises a demodulator 1 3 which may be a direct optical frequency discriminator, for example, a Fabry-Perot Interferometer which converts the frequency variations into intensity variations. Alternatively demodulation may be done electrically after heterodyne detection.

Heterodyne detection requires the provision of a precisely tuned local oscillator which increases the cost and decreases system reliability. In order to overcome this problem a demodulating arrangement as shown in Fig. 3 is proposed.

This demodulator requires the data to be encoded in a return-to-zero format as shown in Fig. 3A wherein each bit has a length T.

The incoming signal is divided into two paths 20 and 21 before recombining at 22 and supplying the output to a photodiode 23. The path 21 is longer than path 20 by an amount which produces a delay of T/2 in the signal propagated along that path. The signal applied to the photodiode 23 will either be the 'zero' frequency alone or both the 'one' and 'zero' frequencies simultaneously. Because of the intrinsic square law characteristic of the photodetection process the difference frequency or d.c. will appear at the output of the photodiode as seen in Fig. 3C. This output is passed through a suitable filter 24 to produce the data output.

The bi-directional data link of Fig. 2 works on the same principle as the above described system. In this case at each end of the transmission line 3 an identical transmitter/receiver unit 14 is provided. Each unit 14 comprises a spectrum stabilised laser 1 5 with wavelength control means 16. The data input is fed via line 1 7 to produce perturbations in the laser bias current to directly frequency modulate the laser. As previously a differentiator (not shown) may be placed in line 1 7 to produce phase modulation. A demodulator 1 8 is also provided coupled to one of the mirrors of its associated laser. The output data is fed along line 19.

At any one time only one of the lasers 1 5 is transmitting along the monomode fibre transmission line 3. The other laser 1 5 is injection locked to the transmitting laser in the same manner as the repeater laser 10 of the Fig. 1 system. Therefore both lasers act as synchronised oscillators and the output from the other mirror of the receiving laser 1 5 is fed directly to its associated demodulator 18.

As it is necessary to provide the possibility of transmission in both directions along the line 3 it is not possible to place an isolator in the line to prevent backward transmissions by the receiving laser which result in ringing between the lasers. In order to overcome this problem one or both lasers 1 5 may be turned off for a short period intermittently. An alternative solution is for the transmitting laser to operate at a higher power than the receiving laser to prevent interaction.

It will be appreciated that the above described systems may be used to transmit digital or analogue signals by frequency or phase modulation. Phase modulation, in particular phase shift keying is a preferred form of modulation as this method avoids slippage of injection locking. In order to maximise repeater spacing the minimum bandwidth should be used; however data transmission rates can be increased by the use of multilevel phase or frequency shift keying.

Claims (9)

1. A laser telecommunication system comprising a spectrum stabilised first laser arranged to produce an output which may be angle modulated and which is coupled to one end of an optical fibre link, a spectrum stabilised second laser coupled to the other end of the fibre link and arranged to produce an output which may be angle modulated, each said laser being arranged so that it can be injection locked by a signal received from the other laser so that it can oscillate in synchronism with said other laser, and means for selecting which laser is to transmit and which to receive.

2. A system as claimed in claim 1, wherein said selecting means comprises means for supplying a higher power to the transmitting laser.

3. A system as claimed in claim 1 or claim 2 wherein the signal to be transmitted modulates the laser bias current either directly or via a differentiator to frequency or phase modulate the laser respectively.

4. A system as claimed in any one of claims 1 to 3, wherein a digital signal is transmitted by frequency or phase shift keying.

5. A system as claimed in any one of claims 1 to 4, wherein at least one of the lasers is a semiconductor laser.

6. A system as claimed in any one of claims 1 to 5 wherein the or each monomode optical fibre section is polarisation holding.

7. A system as claimed in any one of claims 1 to 6, which operates at a centre wavelength of substantially 1.55 ,um.

8. A system as claimed in any one of claims 1 to 7, including one or more repeaters, the or each repeater comprising a laser which is arranged to be injection locked by a received signal.

9. A laser telecommunications system substantially as hereinbefore described with reference to and as shown in Fig. 2 of the accompanying drawings.