GB1565764A

GB1565764A - Optical fibre digital transmission systems

Info

Publication number: GB1565764A
Application number: GB424878A
Authority: GB
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1978-02-02
Filing date: 1978-02-02
Publication date: 1980-04-23
Also published as: DE2902789C2; DE2902789A1; AU4367579A

Description

(54) OPTICAL FIBRE DIGITAL TRANSMISSION SYSTEMS (71) We, STANDARD TELEPHONES AND CABLES LIMITED, a British Company of 190 Strand, London W.C.2. England. do 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: This invention relates to transmitters for optical fibre transmission systems for digital transmission.

Optical fibre digital transmission systems are capable of carrying very many speech channels and/or data channels, even digital television signals, such is the wide bandwidth available. However, it is desirable in such systems to make provision for a low speed channel, in addition to the main high speed digital channels. This low speed channel can be used to provide what is known in telephony as an Engineer Order Wire and/ or for supervisory purposes, e.g. for monitoring the repeaters in a repeated system.

Since these requirements need only very low modulation rates it is undesirable to reduce the high speed digital capacity of the system by reserving a high speed channel for the low speed information.

An attractive optical power source for high speed digital optical links is the gallium arsenide continuous wave laser. These lasers typically exhibit a light output-versuscurrent characteristic which has a threshold below which only a small amount of spontaneous emission is produced and above which lasing occurs. The lasers are modulated by d.c. biassing to the lasing threshold and then adding a somewhat smaller digital modulation current.

According to the present invention there is provided a transmitter for an optical fibre digital transmission system. including a gallium arsenide (GaAs) laser diode having an operating threshold level, means for d.c.

biassing the diode to a level which is a predetermined amount below the threshold level, means for digitally modulating the diode above the threshold level, and means for enveloping modulating the digitally modulated output of the diode at a rate substantially lower than the digital modulation rate.

Unfortunately the lasing threshold in present day diodes varies with both temperature and life, as well as being different between ostensibly similar devices. Some workers have built optical transmitters in which the c.w. lasers are biassed with a fixed d.c. level which must be preset for each laser, but this is completely impracticable for a real system. If the threshold changes and the laser is biassed below threshold, turn-on delay occurs which causes considerable intersymbol interference due to the dependence of the delay on the previous data. Conversely. if the laser is biassed to too high a current then the lasers frequently become extremely noisy and may be permanently damaged through excessive optical power density.

The ideal optical source should provide a normalised optical output for standardised electrical input levels, irrespective of age, temperature and choice of device, and this can be achieved by means of optical feedback'. This is analogous to using feedback around a transistor to ensure constant circuit gain despite variations in transistor gain. It is not possible to monitor the optical output of the laser by looking at the electrical input, so some fraction of the laser output must be monitored, compared with the required output signal, and the electrical drive adjusted appropriately. The light to be monitored may be tapped off as a fraction of the light launched into the fibre, causing a small loss in launched power, or less representatively but more conveniently taken from the rear face of the laser.

The simplest type of feedback is one which merely corrects for changes in threshold and this requires a slow d.c.

feedback only. Unfortunatelv the slope efficiencies of lasers differ enormously. some lasers only requiring 1 mA whilst others require as much as 100 mA of modulation current for full modulation. The slope efficiency can also change with life and, to a lesser degree, with temperature.

One way to provide optimum operation of a GaAs c.w. laser is to use a control system in which a first signal representative of the threshold power is derived from the monitored output of the laser and compared with a threshold reference, a second signal representative of the modulation depth is derived from the monitored output and compared with a modulation reference signal and these two signals are used to produce error signals which can be fed back to the laser driving circuits to control the threshold and modulation power supplied to the laser.

Embodiments of the invention will now be described with reference to the accompanying drawings. in which: Figure 1 illustrates a low rate modulation of the bias applied to a laser diode in which the feedback from the diode is used to control the mean power applied to the diode, and Figure 2 illustrates a low modulation of the bias applied to a laser diode in which the feedback is used to control the modulation depth of the diode.

In the arrangement shown in Figure 1 the light output of a GaAs semiconductor laser diode 1 is coupled into the end of an optical fibre transmission line 2. The laser diode is d.c. biassed to a point just below its threshold current value bv a controlled current source 3. Modulation of the diode is achieved bv superimposing on the bias current a modulation current from a controlled current source 4. The digital signals to be transmitted are applied as control signals to the controlled current source 4. A proportion of the light output of the laser diode 1 is coupled to a photodetector diode 5 which forms part of a feedback circuit.

The photodetector output is fed to one input of an operational amplifier 6 and a reference voltage Vr l(hl is fed to the other input. The output of the amplifier 6 is used to control the bias current source 3. This arrangement is known. To envelope modulate the laser diode output a low speed modulator 7 is coupled to the reference voltage input and the low speed information modulates the reference voltage. This in turn effectivelv modulates the bias current applied to the laser diode.It must be noted that the modulation of the bias current. apart from being at a slow rate. is limited in amplitude so that at one extreme it does not raise the bias current to the threshold level of the laser diode and at the other extreme it does not reduce the total maximum current from sources 3 and 4 together to a level equal to or even only slightly above the threshold current. In other words, the envelope modulation must not degrade the high speed digital modulation of the laser diode to any significant degree. The low speed envelope modulation is readily detectable at a remote receiver by a low bandpass filter. The receiver's automatic gain control (AGC) will eliminate such low speed modulation in the normal data path of the receiver, so the low speed detection must be performed at a point before the AGC takes effect.Furthermore. it is possible to have a portable separate low speed receiver which can be coupled to the optical fibre at intermediate points in the system, e.g. at unattended repeaters or at points where a fault has occurred. such as a break in the fibre. This allows speech communication to be established between an engineer at the transmitter and another engineer in the field.

An alternative arrangement is illustrated in Figure 2. The laser diode 21 is energised by the digital modulation current from controlled current source 22 superimposed on the bias current from a controlled current source 23. Digital modulation is effected by means of switches Si and S for the 1' and '()' binary conditions respectively. These switches control. via respective unity gain amplifiers 24. 25, the inputs to a differential amplifier 26 the output of which is fed to an operational amplifier 27. The operational amplifier also receives a voltage reference V,.,.,ln,) which controls the depth of modulation of the laser diode 21.The switches S1 and S2 which are controlled by the digital information signals. are fed from the output of a feedback amplifier 28 to the input of which is coupled a photodetector diode 29.

The arrangement thus far described controls the depth of digital modulation of laser diode 21. The switch S.. which is closed during the () binary ço dition of the high speed input signals. also feeds a separate operational amplifier 30 the output of which controls the bias current source 23. The arrangement so far described is known. The low speed envelope modulator 31 is coupled to the voltage reference source Vref(m) and envelope modulates the amplitude of the optical '1' level but not the optical '0' level of the laser diode output. i.e. it modulates the absolute amplitude of the digitally modulated output. Again the low speed modulation is readilv extracted bv a low bandwidth receiver inserted into the system at some remote point preceding the use of an AGC.

WHAT WE CLAIM IS: l. A transmitter for an optical fibre digital transmission system. including a gal

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

which merely corrects for changes in threshold and this requires a slow d.c.

feedback only. Unfortunatelv the slope efficiencies of lasers differ enormously. some lasers only requiring 1 mA whilst others require as much as 100 mA of modulation current for full modulation. The slope efficiency can also change with life and, to a lesser degree, with temperature.

One way to provide optimum operation of a GaAs c.w. laser is to use a control system in which a first signal representative of the threshold power is derived from the monitored output of the laser and compared with a threshold reference, a second signal representative of the modulation depth is derived from the monitored output and compared with a modulation reference signal and these two signals are used to produce error signals which can be fed back to the laser driving circuits to control the threshold and modulation power supplied to the laser.

Embodiments of the invention will now be described with reference to the accompanying drawings. in which: Figure 1 illustrates a low rate modulation of the bias applied to a laser diode in which the feedback from the diode is used to control the mean power applied to the diode, and Figure 2 illustrates a low modulation of the bias applied to a laser diode in which the feedback is used to control the modulation depth of the diode.

In the arrangement shown in Figure 1 the light output of a GaAs semiconductor laser diode 1 is coupled into the end of an optical fibre transmission line 2. The laser diode is d.c. biassed to a point just below its threshold current value bv a controlled current source 3. Modulation of the diode is achieved bv superimposing on the bias current a modulation current from a controlled current source 4. The digital signals to be transmitted are applied as control signals to the controlled current source 4. A proportion of the light output of the laser diode 1 is coupled to a photodetector diode 5 which forms part of a feedback circuit.

The photodetector output is fed to one input of an operational amplifier 6 and a reference voltage Vr l(hl is fed to the other input. The output of the amplifier 6 is used to control the bias current source 3. This arrangement is known. To envelope modulate the laser diode output a low speed modulator 7 is coupled to the reference voltage input and the low speed information modulates the reference voltage. This in turn effectivelv modulates the bias current applied to the laser diode.It must be noted that the modulation of the bias current. apart from being at a slow rate. is limited in amplitude so that at one extreme it does not raise the bias current to the threshold level of the laser diode and at the other extreme it does not reduce the total maximum current from sources 3 and 4 together to a level equal to or even only slightly above the threshold current. In other words, the envelope modulation must not degrade the high speed digital modulation of the laser diode to any significant degree. The low speed envelope modulation is readily detectable at a remote receiver by a low bandpass filter. The receiver's automatic gain control (AGC) will eliminate such low speed modulation in the normal data path of the receiver, so the low speed detection must be performed at a point before the AGC takes effect.Furthermore. it is possible to have a portable separate low speed receiver which can be coupled to the optical fibre at intermediate points in the system, e.g. at unattended repeaters or at points where a fault has occurred. such as a break in the fibre. This allows speech communication to be established between an engineer at the transmitter and another engineer in the field.

An alternative arrangement is illustrated in Figure 2. The laser diode 21 is energised by the digital modulation current from controlled current source 22 superimposed on the bias current from a controlled current source 23. Digital modulation is effected by means of switches Si and S for the 1' and '()' binary conditions respectively. These switches control. via respective unity gain amplifiers 24. 25, the inputs to a differential amplifier 26 the output of which is fed to an operational amplifier 27. The operational amplifier also receives a voltage reference V,.,.,ln,) which controls the depth of modulation of the laser diode 21.The switches S1 and S2 which are controlled by the digital information signals. are fed from the output of a feedback amplifier 28 to the input of which is coupled a photodetector diode 29.

The arrangement thus far described controls the depth of digital modulation of laser diode 21. The switch S.. which is closed during the () binary ço dition of the high speed input signals. also feeds a separate operational amplifier 30 the output of which controls the bias current source 23. The arrangement so far described is known. The low speed envelope modulator 31 is coupled to the voltage reference source Vref(m) and envelope modulates the amplitude of the optical '1' level but not the optical '0' level of the laser diode output. i.e. it modulates the absolute amplitude of the digitally modulated output.Again the low speed modulation is readilv extracted bv a low bandwidth receiver inserted into the system at some remote point preceding the use of an AGC.

WHAT WE CLAIM IS: l. A transmitter for an optical fibre digital transmission system. including a gal

lium arsenide (GaAs) laser diode having an operating threshold level, means for d.c.

biassing the diode to a level which is a Predetermined amount below the threshold level, means for digitally modulating the diode above the threshold level, and means for envelope modulating the digitally modulated output of the diode at a rate substantially lower than the digital modulation rate.
2. A transmitter according to claim 1 wherein the means for envelope modulating comprises means for modulating the d.c.

bias applied to the diode.
3. A transmitter according to claim 1 wherein the means for envelope modulating comprises means for modulating the amplitude of the digital modulation of the diode.
4. A transmitter according to claim 3 including a feedback path optically coupled to the diode, the feedback path including means for generating separate error signals for controlling the d.c. bias level and the depth of modulation, the envelope modulation means being connected to modify the error signal controlling the depth of the digital modulation.
5. A transmitter for an optical fibre digital transmission system, substantially as described with reference to Figure 1 or Figure 2 of the accompanying drawings.