GB2273839A

GB2273839A - Electro-optic modulator

Info

Publication number: GB2273839A
Application number: GB9226807A
Authority: GB
Inventors: Richard Edward Epworth
Original assignee: Northern Telecom Ltd
Current assignee: Nortel Networks Ltd
Priority date: 1992-12-23
Filing date: 1992-12-23
Publication date: 1994-06-29
Also published as: GB9226807D0

Abstract

An electro-optic modulator (10) has its capacitance incorporated in the resonant circuit of an RF oscillator. Such a modulator may be supplied with optical power from an unmodulated optical source (13) to form an optical soliton pulse generator. Alternatively the modulator may be supplied with a digitally modulated optical soliton pulse train and the frequency and phase of the RF oscillator locked to that of the clock component of that pulse train to provide retiming and reshaping of its solitons. The RF oscillator may be voltage-controlled. The invention reduces the drive voltage requirements of the RF oscillator. <IMAGE>

Description

ELECTRO-OPTIC MODULATOR This invention relates to an electro-optic modulator, and finds application in the generation of optical solitons and also in the retiming and reshaping of optical solitons.

A paper by M Susuki et al., entitled 'Transform - Limited 14 ps Optical Pulse Generation with 15 GHz Repetition Rate by InGaAsP Electroabsorption Modulator', Electronics Letters 21st May 1992 Vol 28, No 11, pp 1007-8, describes how an electro-absorption optical modulator can be driven with a sinusoidal waveform under conditions which produce an extinction ratio (measured in dB) that varies substantially linearly with bias voltage. An unmodulated beam incident upon such a modulator is converted by its passage through the modulator into a set of pulses of approximately sech2 shape.

The paper explains that this method of soliton generation is very attractive because, apart from the modulator itself, all that is required is an RF voltage source - it requires no (optical) resonators - and it gives a high flexibility in repetition rate.

A paper by M Nakazawa et al entitled '10 Gbit/s Soliton Data Transmission over One Million Kilometres, Electronics Letters 4 July 1991, Vol 27, No 14, pp 1270-2, suggests how a concatenation of lithium niobate Mach Zehnder optical modulators can be used to retime and reshape soliton pulses in their passage down a long distance transmission path in such a way as to circumvent the Gordon - Haus limitation, to restrict the accumulation of amplified spontaneous emission and to counter interaction forces between adjacent solitons.

In both these papers the electro-optic modulator is driven by an externally applied RF signal. In the case of the soliton pulse generator paper, the modulation is an InGaAsP electro-absorption modulator driven with a 15 GHz RF signal that is 6.5V peak to peak. In the soliton retiming and reshaping paper the active element of the modulator is made of lithium niobate, and receives its RF drive from an electronic amplifier.

The amplitude of that drive is not specified in the paper, but the paper does suggest that it was probably slightly insufficient.

The present invention affords a way of driving an electro-optic modulator that does not involve a requirement for the application of a high power RF signal from an external source.

According to the present invention there is provided an electro-optic modulator possessing a reactance which is included in the resonant circuit of an RF oscillator.

A feature of the invention is that the drive voltage requirements for the oscillator, that are necessary in order to provide adequate modulation depth, are reduced in magnitude by virtue of the voltage multiplication effects produced by the Q of the resonant circuit.

There follows a description of electro-optic modulators embodying the present invention in preferred forms. The description refers to the accompanying drawings, in which Figure 1 is a schematic diagram of an optical soliton pulse generator incorporating an electro-optic modulator embodying the invention in a first form, Figure 2 is a schematic diagram of an optical soliton pulse generator incorporating an electro-optic modulator embodying the invention in an alternative form, Figure 3 is a schematic diagram of an optical soliton pulse retiming and reshaping circuit incorporating an electro-optic modulator embodying the invention, and Figure 4 depicts, as a function of time, an illustrative data modulated soliton pulse stream applied to the circuit of Figure 3, the extracted clock component of that pulse stream, the optical attenuation characteristic of the modulator, and the resulting retimed and reshaped output.

Figure 1 depicts a basic form of optical soliton pulse generator consisting of a combined oscillator and modulator 10 receiving an optical input 11 by way of an optical waveguide 12 from an optical source 13 to provide a modulated optical output 14 by way of a further optical waveguide 15. The optical modulator part of the combined oscillator and modulator is an electro-optic modulator having an electrical reactance, typically a capacitance, which is included in the resonant circuit of the oscillator part which is an RF oscillator. In that way advantage can be taken of the voltage multiplication provided by the Q of the resonant circuit to produce an adequate optical modulation depth with a relatively small oscillator rail voltage.In the case of an electro-absorption type modulator the capacitative reactance that is incorporated into the resonant circuit of the RF oscillator is the capacitance of its reverse biassed junction.

In the case of a Mach Zehnder type modulator with an electro-optical element, such as one of lithium niobate, the capacitative reactance that is incorporated into the resonant circuit of the RF oscillator is the capacitance between the electro-optic element electrodes.

If the optical source 13 provides an unmodulated optical output, then operation of the combined oscillator and electro-optic modulator 10 is such as to produce a modulated optical output 14 that has the form of optical soli ton pulses having a pulse repetition frequency equal to that of the natural resonance frequency of the RF oscillator. Such a train of pulses may then be fed to a further modulator (not shown) in order to provide digital modulation of that train.

For many applications it is not satisfactory to have the soliton pulse repetition frequency dictated by a free-running RF oscillator, but instead a specific frequency is required. The soliton pulse generator of Figure 2 has the same components as that of Figure 1, with the additional requirement that the oscillator part of the combined oscillator and modulator 10 is a voltage controlled oscillator (VCO). Additionally the soliton pulse generator of Figure 2 includes a phase sensitive detector 20 providing an output on line 21, which is applied to the voltage control input of the VCO of the combined oscillator and modulator 10.The two inputs to the phase sensitive detector are an electrical signal from the oscillator at the oscillator frequency (and hence also at the soliton pulse repetition frequency) applied on line 22, and an electrical signal at the intended soliton pulse repetition frequency applied on line 23 from an external reference frequency source (not shown).

Turning attention to the optical soliton pulse retiming and reshaping circuit of Figure 3, this employs, with the exception of the optical source 13, the same components as the soliton pulse generator of Figure 2. In this instance the optical input 11, instead of being an unmodulated signal, is a data train of soliton pulses which have become in need of reshaping and retiming in respect of a clock component thereof. The need for retiming and reshaping is typically the result of the pulses having been transmitted over a long distance on a system including one or more optical amplifiers. The optical waveguide 12 includes an optical tap 30 feeding a photodetector 31.The electrical output of the photodetector 31 is fed via an amplifier 32 and a high Q filter 33, typically a SAW filter, tuned to the frequency of the clock component of the data stream, to provide an extracted clock signal for application to the phase sensitive detector on line 23. An adjustable phase delay device 34 is included, either in line 22 or line 23, to compensate for the phase difference that would otherwise exist between the path from the optical tap 30 to the phase sensitive detector 20 that extends via the oscillator 10, and that that extends via the photodetector 31 amplifier 32 and filter 33.

In Figure 4 an illustrative short portion of a soliton pulse data stream which is applied as input 11 to optical waveguide 12 of Figure 3 is depicted by trace 40. The pulses of this portion are in need of retiming and reshaping. In particular the trace 40 shows the optical amplitude as not returning to zero between the move closely spaced pairs of soliton pulses. Trace 41 depicts that clock signal extracted from the soliton data stream, while trace 42 depicts the optical attenuation characteristic (as a function of time) of the combined oscillator and modulator 10, this characteristic varying sinusoidally at the clock frequency between a substantially zero attenuation minimum and a high attenuation maximum. Thus 43 depicts the resulting retimed and reshaped portion of the soliton pulse data stream that emerges on waveguide 15 as optical output 14.

A feature of this form of soliton pulse retiming and reshaping is that the amplitude of the RF drive signal that is applied to the modulator is independent of data-dependent small variations in the amplitude of the clock content of the soliton pulse data stream.

Claims

1. An electro-optic modulator possessing a reactance which is included in the resonant circuit of an RF oscillator.

2. A modulator as claimed in claim 1, wherein the modulator is an electro-absorption modulator whose reverse biassed junction capacitance is included in the resonant circuit of the RF oscillator.

3. A modulator as claimed in claim 1, wherein the modulator is a Mach Zehnder type modulator including an electroded electro-optic element in one of its limbs, and the inter-electrode capacitance of said element is included in the resonant circuit of the RF oscillator.

4. A modulator as claimed in any preceding claim, wherein the RF oscillator is a voltage controlled oscillator (VCO).

5. A modulator as claimed in claim 4, wherein the voltage control input of the VCO is derived from the output of a phase sensitive detector.

6. An optical soliton pulse generator incorporating an unmodulated light source and a modulator as claimed in any preceding claim.

7. An optical soliton pulse generator as claimed in claim 6, wherein the RF oscillator is a VCO whose voltage control input is derived from the output of a phase sensitive detector having a first input derived from the RF oscillator and a second input for the application thereto of a frequency reference signal.

8. An optical soliton pulse generator substantially as hereinbefore described with reference to Figures 1 or 2 of the accompanying drawings.

9. An optical soliton retiming and reshaping circuit including a modulator as claimed in any claim of claims 1 to 5.

10. An optical soliton retiming and reshaping circuit including a modulator as claimed in claim 5, wherein the phase sensitive detector has a first input derived from the RF oscillator and a second input comprising a clock signal extracted from an optical tap of the optical input to the optical soliton retiming and reshaping circuit.

11. An optical soliton retiming and reshaping circuit substantially as hereinbefore described with reference to Figures 3 and 4 of the accompanying drawings.