GB2268851A

GB2268851A - Modulation depth control for FSK/IM optical transmitter

Info

Publication number: GB2268851A
Application number: GB9215197A
Authority: GB
Inventors: Jonathan Paul King
Original assignee: Northern Telecom Ltd
Current assignee: Nortel Networks Ltd
Priority date: 1992-07-17
Filing date: 1992-07-17
Publication date: 1994-01-19
Anticipated expiration: 2012-07-17
Also published as: GB9215197D0; GB2268851B

Abstract

A method of controlling the depth of modulation of an optical source in a frequency shift keyed intensity modulation optical transmitter, the method including the steps of impressing on a digitally modulated drive signal for the source an amplitude modulation at a first frequency less than that of the digital signal, impressing on the digital signal a dither modulation at a second frequency less than that of the first frequency, extracting at 26 from the transmitter output the second harmonic of the first frequency, multiplying the second harmonic with the second frequency and driving therefrom a control signal to control the depth of modulation of the source. <IMAGE>

Description

Modulation depth control for FSK/IM optical transmitter This invention relates to modulation depth control for a frequency shift keyed (FSK) intensity modulated (IM) optical transmitter, such as may be used in a long haul telecommunications transmission system.

The principle of FSK/IM transmitters is that small modulation depth signals imposed on the bias current of an injection laser result in optical frequency changes of the laser output, with a small but negligible amplitude variation. Putting the light through an optical frequency discriminator converts the frequency modulated input into intensity modulation at the output.

By choosing an appropriate optical frequency discriminator, relatively small modulation depth drive signals to the laser bias can be used to provide substantially 100% intensity modulation at the discriminator output. Such an arrangement is disclosed in British patent GB 2 107 147B. Compared to direct intensity modulation of the laser, where large drive current swings are required, this technique yields a much lower chirp transmitter source, which is important for high speed or medium and long haul systems.

Typically use is made of a DFB laser followed by a Mach-Zehnder (M-Z) interferometer optical discriminator.

In a practical system, if the M-Z' characteristic or the centre frequency of the laser drifts the extinction ratio of the IM output is deteriorated, so a compensatory centre frequency control loop is needed. A control scheme which does this has been described in British Patent application 91 06045.9.

The present invention seeks to provide a method and means for controlling the modulation depth in a FSK/IM optical transmitter to compensate for any changes in the laser output frequency vs. injection current characteristic which may be caused by temperature changes and/or by laser ageing. It is important that the laser bias deviation induced by the modulation signal has the correct amplitude, so that the optical frequencies representing the binary '1' and '0' logic levels correspond to the maximum and minimum transmission characteristics of the M-Z interferometer.

Modulation depth can be preset by adjustment of a dc control voltage presented to a variable gain amplifier in the laser drive circuitry.

According to the present invention there is provided an optical transmitter having an optical source adapted to produce a frequency shift keyed (FSK) optical output in response to the application thereto of a drive signal modulated by a digital signal, which FSK output is fed through an optical frequency discriminator to convert the modulation of the optical output from FSK to intensity modulation (IM), the transmitter including a feedback control loop adapted to control the depth of modulation of the optical source, wherein the feedback control loop includes means to impress on the drive signal applied to the optical source an amplitude modulation at a first frequency less than that of the digital signal, the transmitter further including means to impress on the digital signal a dither modulation signal at a second frequency less than that of the first amplitude modulation applied to the drive signal, means for extracting from the transmitter output the second harmonic of the amplitude modulation of the first frequency, means for multiplying the extracted second harmonic with the dither modulation signal and means for deriving from the output of the multiplying means a control signal to control the depth of modulation of the optical source.

An embodiment of the invention will now be described with reference to the accompanying drawings, in which.

Fig. 1 is a block schematic of a modulation depth control arrangement for an FSK/IM transmitter; Fig. 2 depicts the effect of impressing an amplitude modulation on the FSK laser drive signal, and Fig. 2b depicts the characteristic of a Mach-Zehnder interferometer used as an optical frequency discriminator in the arrangement of Fig. 1.

In the arrangement of Fig. 1 incoming binary encoded digital data is applied to a variable gain amplifier 10 the output of which forms the modulated bias current for a DFB injection laser 12. The FSK output of laser 12 is fed into a Mach-Zehnder interferometer 14. One output of the M-Z interferometer is the IM optical signal for transmission, the other output feeds a monitor circuit 16, which comprises a photodiode 18 and amplifier 20.

To provide a modulation depth control to compensate for any changes in laser output frequency vs injection current response which might be caused by temperature changes or laser ageing a feedback control loop is employed. This loop must ensure that the frequency deviation induced by the modulation signal has the correct amplitude, so that the optical frequencies representing "one" and "zero" logic levels correspond to the maximum and minimum transmission of the M-Z. At present modulation depth has been preset by adjusting a dc control voltage presented to the variable gain amplifier 10 in the laser drive chain. The feedback control loop includes a first oscillator 22 running at a frequency, say 6 kHz, which is moderately low in relation to the data frequency, which may be in the giga Hertz range.The output of oscillator 22 is impressed on the dc control voltage of variable gain amplifier 10 to cause a low level amplitude modulation of the laser output, as shown in Fig. 2a. The output of oscillator 22 is also mixed, via a frequency doubler 24, with the output of the monitor circuit 16 in mixer 26 to obtain the second harmonic of the oscillator frequency. This second harmonic will be maximised when the modulation depth is at the correct level. The second harmonic arises as a result of the curvature of the M-Z characteristic at the peaks and troughs, as shown in Fig.2b. In addition to the first oscillator frequency a second oscillator 28, whose frequency is low compared to that of the first oscillator, e.g. a few hundred hertz, is used to impose a dither signal on the data input fed to the laser drive circuitry.This second oscillator frequency is also mixed, in mixer 30, with the second harmonic output of mixer 26. The effect of dithering the data input is to give an optical output as shown in Fig. 2b. The output of the second mixer 30 will vary at the frequency of the second oscillator and will be in-phase with the output of the second oscillator if the modulation depth is too small, as shown in Fig. 2b. This is because when the modulation depth is too small a positive going part of the second oscillator frequency raises the data mean so that the top of the data envelope, which has an in-phase component of the first oscillator frequency, is discriminated on a more non-linear part of the M-Z characteristic which produces more in-phase second harmonic. Conversely, if the modulation depth is too great a negative going part of the second oscillator frequency moves the data mean down, so that the bottom of the data envelope, which has an out-of-phase component of the first oscillator frequency, is discriminated on a more non-linear part of the M-Z characteristic which produces more out-of-phase second harmonic. The control loop is completed by an integrator 32 which integrates the output of mixer 30 to form a control signal which is imposed on the dc control of the variable gain amplifier 10.

As an alternative to using the frequency doubler 24 to feed the output of oscillator 22 to mixer 26, the oscillator 22 can be run at twice the original frequency with its output applied direct to mixer 26 and via a divide-by-two circuit to variable gain amplifier 10.

Claims

1. An optical transmitter having an optical source adapted to produce a frequency shift keyed (FSK) optical output in response to the application thereto of a drive signal modulated by a digital signal, which FSK output is fed through an optical frequency discriminator to convert the modulation of the optical output from FSK to intensity modulation (IM), the transmitter including a feedback control loop adapted to control the depth of modulation of the optical source, wherein the feedback control loop includes means to impress on the drive signal applied to the optical source an amplitude modulation at a first frequency less than that of the digital signal, the transmitter further including means to impress on the digital signal a dither modulation signal at a second frequency less than that of the first amplitude modulation applied to the drive signal, means for extracting from the transmitter output the second harmonic of the amplitude modulation of the first frequency, means for multiplying the extracted second harmonic with the dither modulation signal and means for deriving from the output of the multiplying means a control signal to control the depth of modulation of the optical source.

2. An optical transmitter according to claim 1, wherein the optical source is a DFB injection laser driven by the output of a variable gain amplifier the gain of which is modulated by the digital signal.

3. An optical transmitter according to claim 1 or 2 wherein the drive signal is generated by a voltage controlled variable gain amplifier to which the digital signal is applied, the control signal being in the form of a dc signal imposed on a preset dc control of the variable gain amplifier.

4. An optical transmitter according to claim 1, 2 or 3, wherein the optical frequency discriminator is a Mach-Zehnder interferometer.

5. An optical transmitter including a feedback control loop adapted to control the depth of modulation of the transmitter substantially as described with reference to the accompanying drawings.

6. A method of controlling the depth of modulation of an optical source in a frequency shift keyed intensity modulation optical transmitter, the method including the steps of impressing on a digitally modulated drive signal for the source an amplitude modulation at a first frequency less than that of the digital signal, impressing on the digital signal a dither modulation at a second frequency less than that of the first frequency, extracting from the transmitter output the second harmonic of the first frequency, multiplying the second harmonic with the second frequency and driving therefrom a control signal to control the depth of modulation of the source.

7. A method according to claim 6 wherein yhe optical source is an injection laser and the control signal is applied to control the amplitude of the modulation of the drive current applied to the laser.

8. A method of controlling the depth of modulation of an optical source in a frequency shift keyed intensity modulation optical transmitter substantially as described with reference to the accompanying drawings.