WO2009068082A1

WO2009068082A1 - An optical modulator which uses electrical techniques

Info

Publication number: WO2009068082A1
Application number: PCT/EP2007/062899
Authority: WO
Inventors: Bengt-Erik Olsson
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2007-11-27
Filing date: 2007-11-27
Publication date: 2009-06-04

Abstract

A device (100, 200) for generating an optical modulated signal (150, 250), comprising means (120, 220) for encoding a first data stream (D1) of digital data onto an electrical carrier of a first carrier frequency (f1). The device also comprises means (130) for modulating an optical carrier signal with the encoded electrical carrier, so that a modulated optical signal (150, 250) is generated, the modulated optical signal comprising a center frequency (f0) as well as one positive (f1) and one negative (-f1) sideband signal. The device further comprises means (140) for filtering out one of said sideband signals (- f1, f1). The device may also comprise means for encoding a second data stream (D2) of digital data onto the electrical carrier.

Description

TITLE

An optical modulator which uses electrical techniques.

TECHNICAL FIELD The present invention discloses a device for generating an output optical modulated signal. The device comprises means for encoding a first data stream of digital data onto an electrical carrier of a first carrier frequency, and also comprises means for modulating an optical carrier signal with the modulated electrical carrier so that a modulated optical signal is generated.

BACKGROUND

The use of multi-level phase and amplitude modulated optical signals is becoming more and more attractive due to high bandwidth efficiency and good transmission performance compared to the traditional on-off modulation of optical signals. A required increase in optical data transmission capacity has for a long time been solved by adding new wavelength channels in optical fibres, but this solution will naturally be limited, e.g. by the available bandwidth in the fibre.

It has thus become necessary to improve the utilization of the available optical bandwidth, which has led to an increased interest in methods by means of which it may become possible to increase the spectral efficiency of data channels in optical systems.

Traditionally, optical systems have used simple on-off keying over an optical fibre to transmit data, but a more bandwidth efficient method is to utilize both the phase and the amplitude of the optical carrier to convey information, a technique which in this text will be referred to as quadrature amplitude modulation, QAM.

QAM-modulation in general implies that combinations of bits in an incoming data stream are encoded as symbols in a system with both phase and amplitude significance, such a system often being referred to as an I- and Q- system, i.e. an orthogonal coordinate system with one axis being referred to as the l-axis (horizontal) and the other as the Q-axis (vertical). In such a system, the distance of a symbol from the origin will represent the optical amplitude, and the positive angle from the l-axis will represent the optical phase. In a system with IQ-modulation, a phase shifter is often used in order to introduce and/or maintain a phase shift of ninety degrees between the I- and Q-signals, so that orthogonality is established and maintained.

Existing systems for QAM-modulation in optical systems have a number of drawbacks, among which the following can be mentioned:

• A phase shifter, which is used, needs to be controlled exactly, so that a desired phase shift of, for example, ninety degrees is maintained over all ambient temperatures and all wavelengths, • Maintaining an exact phase shift of, for example, ninety degrees is complicated,

• Existing optical QAM-modulators are usually quite bulky, and may also be difficult to integrate with a light source such as a laser.

SUMMARY

Thus, as has emerged from the description above, there is a need for a solution by means of which an optical modulator can be obtained which does not suffer from the drawbacks of present day systems.

Such a solution is offered by the present invention, in that it discloses a device for generating an output which is an optical modulated signal. The device of the invention comprises means for encoding a first data stream of digital data onto an electrical carrier of a first carrier frequency.

The device also comprises means for modulating an optical carrier signal with the encoded electrical carrier so that a modulated optical signal is generated which comprises at least a first sideband signal, and the device further comprises means for filtering out this first sideband signal, so that the sideband signal may be used by a party which receives the output from the device.

In a preferred embodiment of the invention, the optical carrier signal is modulated by the encoded electrical carrier so that the generated modulated optical signal also comprises a center frequency as well as a positive and a negative sideband signal, and the first sideband signal mentioned above is one of the positive and negative sideband signals.

In a preferred embodiment, the device of the invention additionally comprises means for encoding a second data stream of digital data onto the electrical carrier.

Suitably, the means for encoding the first and/or second data stream onto an electrical carrier of a first carrier frequency will comprise an analogue modulator, and in the case of two data streams, the first and second data streams are modulated into complementary l-and Q-components.

In another embodiment, the means for encoding the first and/or second data stream onto an electrical carrier of a first carrier frequency can however comprise a coder which encodes a digital input sequence into a desired encoded digital output sequence. Such a coder can comprise a digitally phase controlled clock generator for encoding phase modulated data, or a programmable pattern generator which is able to encode output information in both phase and amplitude.

The device of the invention will suitably have a modulated output optical signal which has been modulated according to one of the following modulation methods: • QAM, Quadrature Amplitude Modulation,

• BPSK, Binary Phase shift Keying,

• QPSK, Quadrature Phase shift Keying,

• ASK-QPSK, Amplitude Shift Keying - Quadrature Phase shift Keying,

Or one of the following modulation methods:

• N-QAM, "N" Quadrature Amplitude Modulation,

• N-PSK, "N" Phase shift Keying, where: N is an arbitrary positive integer.

The invention also discloses a method for modulating an optical carrier signal with an encoded electrical carrier.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be described in more detail in the following, with reference to the appended drawings, in which

Fig 1 shows a first embodiment of the invention, and Fig 2 shows a second embodiment of the invention, and Fig 3 shows a table for use with the embodiment of fig 2,

Fig 4 shows a schematic flow chart of a method of the invention.

DETAILED DESCRIPTION

Fig 1 shows a block diagram of a first embodiment 100 of the invention. The invention will in fig 1 and in the following be described with the aid of a device which has as its output an optical signal with a centre frequency as well as a positive and a negative sideband. It should however be pointed out that this is merely an example which is used in order to facilitate the reader's understanding of the invention. The invention can equally well be applied to a device which has as its output an optical signal which, for example, comprises a centre carrier with one sideband only, or an optical signal which has two or more sidebands with or without a centre carrier.

As has been stated above, a purpose of the invention is to obtain a device which can provide a modulated output optical signal in a manner which is an improvement over the prior art. To this end, a principle behind the present invention is to encode incoming data onto an electrical carrier with a frequency fi which suitably is higher than or equal to the symbol rate of the incoming data, and to subsequently modulate an optical carrier with the encoded electrical carrier.

Returning now however to the embodiment shown in fig 1 , the modulation of the optical carrier will generate an optical spectrum which comprises a centre carrier which corresponds to the frequency of the light source of the optical carrier, usually a laser, which is modulated by the encoded electrical carrier. The optical spectrum of this particular device will also comprise at least two modulated side band carriers, which have centre frequencies which correspond to the frequency of the light source ± the frequency of the light source. If the frequency of the light source is referred to as fo, the side band carriers will thus be centred on frequencies fo ± U- Both of these side bands will contain the information which was encoded onto the electrical carrier, and in order to obtain that information, it will be accordingly be sufficient to retain only one of said side band signals, f₀ ± f i .

Hence, a device of the invention will also comprise means for filtering out and retaining one of the sideband signals, so that this signal may be used by a party which receives the output from the device. Said receiving party will then be able to access all of the information which was encoded onto the electrical carrier of the inventive device.

Returning now to the block diagram shown in fig 1 , the device 100 comprises a source 110 for an electrical carrier at the frequency f-i , a suitable such source being a Local Oscillator, an LO. The output signal from the LO 110 is used as input to a modulator 120, which also has as another input a first data stream D₁ (t), and in a preferred embodiment, a second data stream D₂(t) is also used as input to the modulator 120.

Suitably, in the modulator 120, the first and second data streams Di(t) and D₂(t) are modulated into complementary l-and Q-components, i.e. two signals which are phase shifted ninety degrees from each other. The modulator 120 is preferably an analogue modulator, so that the two data streams Di(t) and D₂(t) are also analogue signals, but digital input signals may be used if they are passed through a digital to analogue converter before being input to the modulator 120.

As is shown in fig 1 , the device 100 also comprises a light source modulator 130, suitably a component which modulates a laser with the centre frequency f₀. The component 130 can incorporate a light source such as a laser, in combination with an external modulator such as, for example, a Mach-Zender modulator or an electro-absorption modulator, or the modulator 120 can alternatively directly modulate laser current or voltage of the component 13O.t As explained previously, and as also indicated in fig 1 , the output signal from the light modulation component 130 will comprise a centre frequency component which is centred at the centre frequency of the modulated light source, i.e. f₀, and the output signal will also comprise a first and a second sideband signal, centred at frequencies fo±f-i, where fi is the centre frequency of the original electrical carrier from the LO 110. The output signal from the modulator 130 is used as input to a means 140 for filtering out one of these sideband signals, so that that signal may be used by a party which receives the output signal 150 from the device 100. By using, for example, a so called Mach Zender modulator it would be possible to bias the modulator in order to suppress the centre carrier, which would simplify the subsequent optical filtering. Suitably, the filtering means 140 is an optical bandpass filter, BPF, which can for example be a so called interference filter, a free space grating filter, a fibre Bragg grating filter, or a Mach-Zehnder interferometer filter. Naturally, combinations of these filters may also be used.

Fig 2 shows a second embodiment of a device 200 of the invention. Components etc which correspond to those in fig 1 have been given the same reference numerals as those in fig 1.

A difference between the device 200 of fig 2 and the device 100 of fig 1 is that the device 200 replaces the analog modulator 120 of the device 100 of fig 1 with a control device 220 which outputs an encoded digital sequence which corresponds to a desired phase and/or amplitude modulation of the input signals.

The control device 200 can be designed in a variety of ways, among which mention can be made of such techniques as a digitally phase controlled clock generator which encodes phase modulated data, or the control device 220 can be a programmable bit pattern generator which can encode both the phase and the amplitude of the output electrical signal. In these cases, a digital sequence with subsequent electrical and/or optical filtering can to some extent synthesize an analog signal including amplitude modulation.

With further reference to the control device 220, it can be pointed out that the carrier frequency fi of the LO of the device 100 may be calculated in the following manner: Assume a digital sequence of slots where N slots constitutes a symbol and k slots represents a period of symbols. Further, if t equals the time duration of each slot in the sequence, then the carrier frequency of the output signal becomes 1/(N ^* t) and the baud-rate of the output signal becomes 1/(N^* k^* t). For k = 1 the carrier frequency equals the baud-rate, which maximizes the bandwidth utilization of the electrical components. However, using k > 1 significantly simplifies optical filtering. It should also be pointed out that although the device 200 is shown as having two inputs, Di(t) and D₂(t), this is only an example, the device 200 can have only one input, or it can, alternatively, have more than two inputs. The inputs to the device 200 should be digital, but of course, analogue to digital converters can be used if analogue inputs signals are utilized.

Fig 3 shows a table with some possible inputs to the devices 100/200, as well as examples of the corresponding modulated outputs using different modulation techniques. It can be pointed out that three of the modulation techniques of fig 3 only use phase information, i.e. BPSK, QPSK and 8-PSK, while ASK-QPSK and 12-QAM provide an output signal which has both phase and amplitude information.

Fig 4 shows a rough flow chart of some of the steps in a method 400 of the invention. Steps which are options or alternatives are shown with dashed lines.

Thus, the inventive method 400 may be used for generating an output optical modulated signal from a first input data stream, and the method comprises, step 410, encoding said first data stream of digital data onto an electrical carrier of a first carrier frequency U- The method 400 also comprises, as shown in step 415, modulating an optical carrier signal with the encoded electrical carrier so that a modulated optical signal is generated, the modulated optical signal comprising a centre frequency (fo) as well as one positive (fi) and one negative (-fi) sideband signal.

The method 400 further comprises, as shown in step 420, filtering out one of the sideband signals, so that that signal may be used by a receiving party.

As indicated in step 425, the method 400 may additionally comprise the step of encoding a second data stream D₂ of digital data onto the electrical carrier. As shown in step 430, the method may comprise encoding the first and/or second data stream onto an electrical carrier of a first carrier frequency by means of an analogue modulator such as the one 120 shown in fig 1. In that case, as shown in step 445, the method may comprise modulating the first and second data streams into complementary l-and Q-components.

As an alternative to an analogue modulator, the method 400 may comprise, step 435, encoding the first and/or second data stream D₁ZD₂ onto an electrical carrier of a first carrier frequency by means of a coder such as the one 220 of fig 2, which encodes a digital input sequence into a desired encoded digital output sequence. Such a coder may comprise, as shown in step 450, the use of a digitally phase controlled clock generator for encoding phase modulated data. Alternatively, as shown in step 455, the coder may comprise the use of a programmable pattern generator which is able to encode output information in both phase and amplitude.

According to the method 400, the generated modulated optical signal may be modulated with one of the following modulation methods:

• QAM, Quadrature Amplitude Modulation,

• BPSK, Binary Phase shift Keying,

• QPSK, Quadrature Phase shift Keying,

• ASK-QPSK, Amplitude Shift Keying - Quadrature Phase shift Keying or one of the following:

• N-QAM, "N" Quadrature Amplitude Modulation,

• N-PSK, "N" Phase shift Keying,

where N is an arbitrary positive integer. In conclusion, the invention provides an improved device and method for generating modulated optical signals, where the output optical signal generated by the inventive method or device is equivalent to a modulated optical signal generated by an optical modulator, but with improvements when it comes to robustness of design and cost effectiveness, as well as allowing for the use of electro-absorption modulators or directly modulated light (e.g. laser) sources.

The invention is not limited to the examples of embodiments described above and shown in the drawings, but may be freely varied within the scope of the appended claims. For example, the means for encoding a data stream of digital data onto an electrical carrier of a first carrier frequency may vary from the examples described above and shown in the drawings without departing from the scope of the present invention. As an example of this, mention can be made of the use of a digital-to-analogue converter instead of the modulator 120 of the device 100 shown in fig 1 and described above. This is of course under the assumption that the data stream or streams D₁/D₂ which are input to the device 100 are also digital.

Similarly, the means used for modulating an optical carrier signal with the encoded electrical carrier may also vary from the examples described above and shown in the drawings without departing from the scope of the present invention.

Also, in a further possible embodiment of the invention, it would be possible to place a filter at the input of the optical modulator 130 in order to "pre- compensate" for impairments or imperfections in the transmission system, such as for example chromatic dispersion or bandwidth limitations.

Such filtering can of course be carried out either in the analogue or in the digital domain.

Claims

1. A device (100, 200) for generating an output which is an optical modulated signal (150, 250), said device comprising means (120, 220) for encoding a first data stream (D-O of digital data onto an electrical carrier of a first carrier frequency (f-i), the device also comprising means (130) for modulating an optical carrier signal with the encoded electrical carrier so that a modulated optical signal (150, 250) is generated, said modulated optical signal comprising at least a first sideband signal, the device (100, 200) being characterized in that it further comprises means (140) for filtering out said first sideband signal (-f-i, fi), so that the sideband signal may be used by a party which receives the output from the device.

2. The device of claim 1 , in which the optical carrier signal is modulated by the encoded electrical carrier so that the generated modulated optical signal comprises a center frequency (f₀) as well as a positive (f-i) and a negative (- fi) sideband signal, said first sideband signal being one of the positive and negative sideband signals.

3. The device (100, 200) of claim 1 or 2, additionally comprising means for encoding a second data stream (D₂) of digital data onto said electrical carrier.

4. The device (100) of any of the previous claims, in which the means (120) for encoding the first and/or second data stream onto an electrical carrier of a first carrier frequency comprises an analogue modulator (120).

5. The device (100) of claim 4, in which the first and second data streams (D-i, D₂) are modulated into complementary l-and Q-components.

6. The device (200) of any of claims 1-3, in which the means (220) for encoding the first and/or second data stream (D-i, D₂) onto an electrical carrier of a first carrier frequency comprises a coder (220) which encodes a digital input sequence into a desired encoded digital output sequence.

7. The device (200) of claim 6, in which said coder (220) comprises a digitally phase controlled clock generator for encoding phase modulated data.

8. The device (200) of claim 6, in which said coder (220) comprises a programmable pattern generator which is able to encode output information in both phase and amplitude.

9. The device (100, 200) of any of the previous claims, in which said generated modulated optical signal is modulated according to one of the following modulation methods:

• QAM, Quadrature Amplitude Modulation,

• BPSK, Binary Phase shift Keying,

• QPSK, Quadrature Phase shift Keying,

• ASK-QPSK, Amplitude Shift Keying - Quadrature Phase shift Keying

10. The device (100, 200) of any of claims 1-9, in which said generated modulated optical signal is modulated according to one of the following modulation methods:

• N-QAM, "N" Quadrature Amplitude Modulation. • N-PSK, "N" Phase shift Keying where N is a positive integer.

11. A method (400) for generating an output which is an optical modulated signal (150) from a first input data stream (D₁, D₂), the method (400) comprising the step (410) of encoding said first data stream of digital data onto an electrical carrier of a first carrier frequency (f-i), and also comprising the step (415) of modulating an optical carrier signal with the encoded electrical carrier so that a modulated optical signal is generated, said modulated optical signal comprising at least a first sideband signal, the method being characterized in that it further comprises the step (420) of filtering out said first sideband signal, so that the first sideband signal may be used by a receiving party.

12. The method of claim 1 , according to which the optical carrier signal is modulated by the encoded electrical carrier signal so that the generated modulated optical signal comprises a centre frequency (fo) as well as a positive (f-i) and a negative (-fi) sideband signal, said first sideband signal being one of the positive and negative sideband signals,

13. The method (400) of claim 11 or 12, additionally comprising the step (425) of encoding a second data stream (D₂) of digital data onto said electrical carrier.

14. The method (400) of any of claim 11-13, comprising the step (430) of encoding the first and/or second data stream onto an electrical carrier of a first carrier frequency by means of an analogue modulator (120).

15. The method (400) of claim 113, comprising (445) modulating the first and second data streams into complementary l-and Q-components.

16. The method (400) of any of claims 13-15, comprising the step (435) of encoding the first and/or second data stream (Di/D₂) onto an electrical carrier of a first carrier frequency by means of a coder (220) which encodes a digital input sequence into a desired encoded digital output sequence.

17. The method (400) of claim 16, comprising (450) the use of a digitally phase controlled clock generator for encoding phase modulated data in said coder (220).

18. The method (400) of claim 16, comprising (455) the use of a programmable pattern generator which is able to encode output information in both phase and amplitude in said coder (220).

19. The method of any of claims 13-18, according to which said generated modulated optical signal is modulated with one of the following modulation methods:

• QAM, Quadrature Amplitude Modulation,

• BPSK, Binary Phase shift Keying,

• QPSK, Quadrature Phase shift Keying,

• ASK-QPSK, Amplitude Shift Keying - Quadrature Phase shift Keying.

20. The method of any of claims 13-18, according to which said generated modulated optical signal is modulated with one of the following modulation methods:

• N-QAM, "N" Quadrature Amplitude Modulation. N-PSK, • "N" Phase shift Keying, where "N" is a positive integer.