This invention relates to an optical modulator with a pre-determined frequency chirp and more especially, although not exclusively, to an electro-optic Mach-Zehnder optical modulator or directional coupler with a pre-determined frequency chirp for use in an optical communications system.
As is known chromatic dispersion is a fundamental property of any waveguiding medium, such as for example the optical fiber used in optical communications systems. Chromatic dispersion causes different wavelengths to propagate at different velocities and is due to both the properties of the material medium and to the waveguiding mechanism.
In a communications system it is fundamental that modulation onto a carrier wave of a stream of digital or analogue data to be communicated causes diversification of the frequency of the carrier into one or more side-bands. Chromatic dispersion in a long optical fiber therefore causes progressive deterioration of the data with distance as the side-bands become phase shifted relative to each other. Chromatic dispersion has the effect of broadening or spreading pulses of data which limits the operating range and/or operating data rate of an optical fiber communications system.
In optical communications it is known to modulate an optical carrier using (i) direct modulation of the optical source, most typically a semiconductor laser, or (ii) external modulation in which the optical source is operated continuously and its light output modulated using an external modulator. In direct modulation the drive current to the laser is modulated thereby changing the refractive index of the active region which produces the required intensity modulation of the light output and additionally an associated optical frequency modulation. The associated optical frequency modulation is known as chirp. Quantitatively, the chirp parameter ax is defined by the expression:
where is I is the intensity, ∂φ/∂t the rate of change of optical phase φ and ∂I/∂t the rate of change of intensity. Laser chirp further limits the operating range and/or data rate in optical communications due to chromatic dispersion. Since semiconductor lasers are generally prone to chirp strongly it is preferred to use external modulation, particularly using electro-optic interferometric modulators, in long-haul high bit rate intensity-modulated optical fiber communications. A particular advantage of external modulators, particularly Mach-Zehnder modulators, are that (i) their chirp is low or zero, (ii) they can operate at much higher modulation frequencies (in excess of 100 GHz has been demonstrated), (iii) their light/voltage characteristic is well defined and has an odd-order symmetry which eliminates even-order harmonic distortion products and (iv) since the light source is run continuously at high stable power its light output is high and has spectral purity making it ideally suited to Wavelength. Division Multiplex (WDM) systems.
Although optical modulators can modulate an optical signal with zero chirp and thereby minimize the effect of optical fiber chromatic dispersion, the operating range and/or data rates of long-haul fiber-optic communications is still limited by chromatic dispersion. To overcome this problem and to give optimum system performance it has been proposed to apply, using the modulator, a small and well controlled negative chirp to compensate for the fiber dispersion (A H Gnauk et al “dispersion penalty reduction using optical modulators with adjustable chirp” IEEE Photon. Technol. Lett. vol. 3 (1991)). Negative chirp is obtained when a rising light level is combined with an optical frequency down-shift due to a net refractive index increase in the modulator (higher refractive index leads to a slower propagation which leads to an increased phase lag and lower frequency) and vice versa. The optimum value for the negative chirp parameter depends on the type and length of the optical fiber and is typically in the range α=−0.5 to −1.0.
The method of imparting negative chirp depends on the type of modulator. Modulators can broadly be characterized as those which are electro-absorptive or electro-refractive in nature.
Electro-absorptive devices utilize a change of material transparency near the bandgap wavelength of a semiconductor material and provide simple ON/OFF gating with non-linear characteristic. Since light is absorbed in a reverse-biased junction-region they are prone to electrical avalanching with potential for run-away at high optical power. There are powerful electro-refractive effects associated with the electro-absorption, which results in a high degree of chirp. They are also highly wavelength specific.
Electro-refractive, often termed electro-optic, modulators use an electric-field induced refractive index change that is a property of certain materials. They are usually based on interferometers and can utilize monolithic, planar, optical guided-wave technology to confine the light to the vicinity of the modulating electric field for distances of up to several centimeters so that the rather weak electro-optic effects can accumulate. Light is not absorbed in the OFF state but rather it is re-routed to an alternative port. Optical modulators of this class, which includes directional couplers, are of interest, not only for modulation, but also for optical switching and for signal processing in optical communications systems.
The predominant type of electro-optic optical modulator uses the Mach-Zehnder interferometer configuration as shown schematically in
As is known, electrically induced relative phase-shifts of +90° between the arms 4, 6 cause the light to switch wholly to one or other of the two outputs 10, 12 upon recombination in the recombiner 8. The light transmission versus modulation voltage Vmod response has a periodic, raised-cosine form.
Intensity-modulation arises from the action of the recombiner 8 on the difference between the phase modulation on the different arms 4, 6 of the interferometer. Any net phase modulation at the outputs 10, 12 arises from that which they have in common and is the same at both outputs. The chirp parameter for a Mach-Zehnder modulator is defined for small excursions about the near-linear (50:50) working point by:
where VL1 is the voltage length product for the first waveguide arm 4 and VL2 is the voltage length product for the second waveguide arm 6. The voltage length product includes sign.
From a limited source of total phase modulation the differential and common phase modulation components are in competition. Consequently an intensity modulator with residual phase modulation (chirp) will be less efficient in other respects than a comparable zero-chirp device.
As is now described, a Mach-Zehnder modulator can be operated in different ways. In a first drive method, termed Single-Sided Drive, a single RF modulating drive voltage Vmod is applied to the modulation electrode of one arm only. This gives a chirp parameter of ±1. The RF drive voltage can be considered as being equivalent to a differential voltage of ±Vmod/2 which is superposed on a common level of Vmod/2 and results in the chirp parameter being non zero. Intensity modulation is proportional to Vmod and the RF power required to drive the modulator is proportional to V2mod.
In a second drive method, termed dual-drive push-pull, independent, equal and opposite RF drive voltages of ±Vmod/2 are applied respectively to the two arms. This drive method yields zero chirp and an intensity modulation proportional to Vmod. The RF drive power required is proportional to V2mod/4+V2mod/4—i.e. half that of a single-sided drive.
In a third drive method, termed Series Push-Pull, the drive electrodes of the two arms are series-connected and driven with a single RF drive voltage Vmod. Half the drive voltage appears across each arm, and they work in antiphase to give the same intensity modulation as both of the above drive methods but with no chirp. The RF power requirement is the same as that of the single-sided drive but the modulator will have about twice the bandwidth since the capacitance presented to the RF source is halved.
Finally, in a fourth drive configuration known as Parallel Push-Pull the drive electrodes of the two arms are cross-connected in parallel and driven from a single RF source drive voltage Vmod/2. In this configuration the arms work in antiphase to give the same intensity modulation as the drive methods described above with no chirp. The RF power requirement for this drive method is now only one quarter of that of the single-sided method. However the capacitance presented to the RF source is double that of the single-sided drive so the modulator will have about half the bandwidth.
Table 1 below summarizes, for the different drive methods described, their chirp parameter, bandwidth and power. In the table all the figures are normalized to the single-sided drive method. It is worth noting that the required drive-voltage and the bandwidth can be traded against each other in an electro-optic modulator design since both are inversely proportional to the length of the drive electrode. However, in terms of the ratio of Bandwidth to Power (a Figure of Merit) a chirp-factor of unity will always cost a factor of two.
A particularly preferred form of modulator for use in optical communication is a Mach-Zehnder modulator fabricated in GaAs/AlGaAs. This type of modulator, for reasons of fabrication, has an inherent built-in electrical back-connection between the two waveguide arms in the form of an n-type doped semiconductor material just beneath the waveguides which is necessary to confine the applied electric field to the guided-wave regions. Thus, the native drive method of GaAs/AlGaAs electro-optic modulators is series push-pull and consequently such a modulator design cannot, without modification, impart chirp.
A development of the above type of optical modulator which is particularly preferred in high speed optical communications is a traveling-wave GaAs/AlGa—As electro-optic modulator. As is known, this type of modulator is a Mach-Zehnder modulator in which the modulation electrode is segmented into a number of electrodes that are disposed along the length of each waveguide arm. The modulating voltage is applied to the electrode segments using a coplanar transmission line from which the electrodes depend and propagates in the form of a traveling RF wave in the same direction as the optically guided wave. The electrode segments in turn provide capacitive loading to the transmission line which thereby acquires slow-wave properties. By appropriate selection of the loaded line, the phase velocity of the traveling RF modulating voltage and the group velocity of the optically guided wave can be precisely matched such that the modulation accumulates monotonically over the length of the waveguiding regions. This results in a much higher degree of optical modulation than is otherwise possible with a standard Mach-Zehnder modulator. Like standard Ga—As/AlGaAs electro-optic modulators these devices have an inherent back-connection between the two arms and are consequently driven in is series push-pull and cannot impart chirp.
Whilst it would, in theory, be possible to apply different modulating drive voltages to the two arms to impart a desired chirp, in practical applications, especially the highest bit rate communications systems, it is impractical and undesirable to do so. For example, separate modulating drive voltages requires two well-matched RF sources or a very well-balanced RF splitter which is impracticable at very high bit rates of tens of giga bits per second. Additionally, the use of separate drive voltages in a very high frequency travelling-wave structure is impractical since it would require dual transmission-drive lines which would require the modulator to be much larger to prevent coupling of the drive signals between the lines. Such coupling would compromise the flatness of the modulator's frequency response.
It has also been proposed to asymmetrically displace the modulating electrodes relative to the waveguide arms in a lithium niobate Mach-Zehnder modulator to imbalance the electro-optic efficiency between the arms and so impart a fixed amount of chirp (P Jiang and A O'Donnell “LiNbO3 Mach-Zehnder Modulators with fixed Negative Chirp”, IEEE Photonics Tech. Lett., Vol. 8 (10), 1996). As is known, in a lithium niobate modulator it is the fringing electric fields from the co-planar electrodes which are placed adjacent to the in diffused waveguides which gives rise to the electro-optic effect. This technique of imparting chirp is only appropriate to modulators in which the modulating electrodes are not inherently in a fixed alignment with the optical waveguides and is consequently not appropriate to GaAs modulators in which the electrodes and waveguides possess an inherent alignment due to the fabrication process.
A need exists therefore for an optical modulator which is capable of imparting a pre-determined amount of frequency chirp, preferably between zero and ±1, which in part alleviates the limitations of the known devices. The present invention has arisen in an endeavor to provide a GaAs/GaALAs Mach-Zehnder electro-optic modulator which is capable of imparting a pre-determined frequency chirp.
According to the present invention an optical modulator for producing a modulated optical output having a pre-determined frequency chirp comprises: optical splitting means for receiving and splitting an optical input signal to be modulated into two optical signals to pass along two waveguide arms made of electro-optic material; optical combining means for receiving and combining the two optical signals into said modulated optical output; at least one electrode pair associated with each waveguide arm, said electrode pairs being electrically connected in series such as to modulate the phase of said optical signals in anti-phase in response to a single electrical signal applied thereto; characterized by a capacitive element connected to the electrode pair of one arm such as to modify the division of the single electrical signal such that the magnitude of the electrical signal across the electrode pair of one arm is different to that across the electrode pair of the other arm thereby imparting the predetermined frequency chirp in the modulated optical output.
The provision of the capacitive element enables the optical modulator of the present invention to achieve a chirp parameter of between 0 and ±1 and can be considered as being driven in a manner which is intermediate between a single-sided and push-pull drive configuration.
It will be appreciated that the provision of a capacitive element to impart a pre-determined frequency chirp can be applied to any electro-optic device having two or more waveguides in which the refractive index of one waveguide is altered relative to that of the other waveguide in response to an electrical signal. As such the present invention also applies to other forms of optical modulators and more especially to a directional coupler when it is operated as a modulator rather than a switching device.
Thus according to a second aspect of the invention an optical modulator for producing a modulated optical output having a predetermined frequency chirp comprises: two optical waveguides of electro-optic material which are located adjacent to each other such as to allow optical coupling between the waveguides and at least one, electrode pair associated with each optical waveguide, said electrode pairs being electrically connected in series such as to de-synchronize the coupling between the waveguide in anti-phase in response to a single electrical signal applied to the electrode pairs; characterized by a capacitive element connected to the electrode pair of one waveguide such as to modify the division of the single electrical signal such that the magnitude of the electrical signal across the electrode pair of one waveguide is different to that across the electrode pair of the other waveguide thereby imparting a predetermined frequency chirp in the optical output.
Advantageously the capacitive element is connected in parallel with the electrode pair of said arm and the single electrical signal is applied to the electrode pairs in a series push-pull configuration. Alternatively the capacitive element is connected in series with the electrode pair of said arm and the electrical signal is applied to the electrode pairs in a parallel push-pull configuration.
The present invention applies to both lumped and traveling-wave implementations. Thus one embodiment comprises a plurality of electrode pairs positioned along each waveguide arm; a respective capacitive element connected to each electrode pair of one arm and a transmission line associated with each arm to which the electrode pairs are electrically connected, wherein the electrode pairs are positioned such that the phase velocity of the electrical signal as it travels along the transmission line is substantially matched to the optical group velocity of the optical signals.
In a preferred implementation, the optical modulator is fabricated in Ill-V semiconductor materials such as GaAs and AlGaAs. Alternatively it can be fabricated in any electro-optic medium.
Conveniently the, or each, capacitive element comprises an additional electrode pair which is provided across a material layer used to guide the optical signals in the modulator and wherein said additional electrode pair is located on a region of said material such that it does not substantially affect the phase of optical signal passing through the associated waveguide arm.
According to a third aspect of the invention. An optical modulator for producing a modulated optical output signal having a predetermined frequency chirp comprises: optical splitting means for receiving and splitting an optical input signal to be modulated into two optical signals to pass along two waveguide arms made of electro-optic material; optical combining means for receiving and combining the two optical signals into said modulated optical output; a plurality of electrode pairs associated with each waveguide arm and positioned along each waveguide arm for differentially modulating the phase of light passing along one waveguide arm relative to that of the other waveguide arm in response to a single electrical signal applied to the electrode pairs and a transmission line associated with each arm to which these electrode pairs are electrically connected, wherein respective electrode pairs on each waveguide arm are electrically connected in series and are connected to the transmission line such that the phase velocity of the electrical signal as it travels along the transmission line is substantially matched to the optical group velocity of the optical signals; characterized by one or more selected electrode pairs being displaced from its associated waveguide such that the or each electrode pair does not substantially affect the phase of the optical signal such as to obtain a the pre-determined chirp in the modulated optical output.
Conveniently one electrode of each selected electrode pair is laterally displaced relative to its associated waveguide such that the phase of the optical signal passing through said waveguide is substantially unaffected by the displaced electrode but wherein the electrical properties of the electrode pair are substantially identical to those of other electrode pairs which have not been displaced.
Preferably the optical modulator is fabricated in a III-V semiconductor material such as GaAs and AlGaAs. Alternatively it can be fabricated in any electro-optic medium.
In order that the invention may be better understood three optical modulators in accordance with the two aspects of the invention will now be described by way of example only with reference to the accompanying drawings in which:
To assist in understanding the optical modulators of the present invention it is helpful to firstly describe the known Mach-Zehnder optical modulator as fabricated in Ga—As/AlGaAs. A sectional end view through the line ‘AA’ of
Since it is intended to drive the modulator using a series-push-pull method, it is required that the back plane electrode, which is constituted by a region 44 of the conductive n-doped AlGaAs layer 24, is free to float to the mid-point of the RF modulating voltage and is not pinned to a ground potential. To ensure this is the case the two trenches 46, 48 are etched through the layers 24, 26, 28 and run parallel with the axis of the waveguide arms. To ensure good electrical isolation of the backplane electrode 44 the isolation trenches 46, 48 are etched a small distance into the semi-insulating GaAs substrate 22. Electrical connection to the modulator electrodes 40, 42 is made by stranded thin film metal structures 40a, 42a in the conducting metallization layer 30, which form air bridges over the isolation trenches 46, 48 to respective modulation drive voltage lines 40b, 42b. As shown in
Referring to
Referring to
Referring to
As can be seen from
Accordingly the electro-optic phase shifts applied to the optical signal passing along the first (right hand in
Referring to
where L1 is the length of the electrodes 40, 62, L2 is the length of the electrode 42, C the capacitance per unit length for the modulating electrodes 40, 42 and Cg the capacitance per unit length of the electrode 62. As is noted from equation 3 no chirp will be imparted when Cg=0 and this is irrespective of the relative lengths of the modulating electrodes L1, L2. This is because the optical modulator is self balancing with regard to the electrode length: a shorter modulating electrode has less capacitance and so, in the absence of Cg, acquires a greater proportion of the modulating RF voltage which thereby exactly compensates for a shorter length. The sign of the chirp is dependent upon the slope of the light/voltage characteristic and is positive at one of the two complementary outputs while it is negative at the other. The degree of chirp is selected primarily by means of the width of the passive element. In effect the additional capacitive element means that the modulator is driven in a way which is intermediate: between a single sided and push-pull configuration and only requires a single RF modulating drive voltage.
Referring to
Referring to
It will be appreciated by those skilled in the art that modifications can be made to the optical modulator described which are within the scope of the invention. For example whilst it is preferable to fabricate the modulator in GaAs/AlGaAs it can be fabricated in other III-V semiconductor materials or other electro-optic materials using appropriate fabrication techniques.
Furthermore, whilst the present invention particularly concerns an electro-optic optical modulator it will be appreciated that the provision of the capacitive element to impart a pre-determined frequency chirp can be applied to other electro-optic devices having two or more waveguides in which the refractive index of one waveguide is altered relative to that of the other waveguide in response to an electrical signal. For example it is envisaged to apply the invention to an electro-optic directional coupler when it is operated as a modulator rather than a switching device. In such a device the two waveguides are located closely adjacent to each other such as to allow optical coupling between them. Electrodes are provided on each waveguide and are such that the application of the electrical signal to the electrodes in a push-pull configuration results in a de-synchronizing of the coupling between the two waveguides due to the relative change in refractive index between the waveguides. This de-synchronizing results in a modulation of an optical signal passing along the or each waveguide. In accordance with the present invention a passive capacitive element is connected to the electrodes of one waveguide such as to modify the division of the electrical signal such that the magnitude of the electrical signal on one waveguide is different to that of the electrode of the other waveguide thereby imparting a pre-determined frequency chirp to the optical signal.
It will be further appreciated that whilst the capacitive element is described as being connected in parallel with the electrodes of one waveguide when the device is drive in series push-pull configuration it can alternatively be connected in series with the electrodes of one waveguide when using a parallel push-pull drive configuration. Furthermore it is also envisaged to use a variable capacitive element, such as an integrated varicap or varactor diode, such that the frequency chirp can be selectively adjusted by the application of an appropriate d.c. bias voltage.
Referring to
For modulators having a total of N active and dummy electrodes of which M have a push-pull configuration and N-M have a single sided drive arrangement the chirp parameter is given by:
Thus for the embodiment illustrated, in which N=5 and M=1, a chirp parameter of ±0.6667 is obtained. A particular advantage of this arrangement is that because the dummy electrodes have been created by merely displacing the ground side electrode away from the waveguide, electrically the arrangement is still essentially identical to that of a standard push-pull arrangement. Since the dummy electrodes discard half the RF modulating drive potential by dropping it across a non-active, dummy, waveguide section the drive voltage necessary to operate the modulator will increase. However since electrically the modulator is equivalent to a standard push-pull arrangement it retains all the benefits of its enhanced bandwidth. The provision of applying selective chirp is therefore only at the expense of a penalty in increased drive voltage rather than of reduced bandwidth as with the first invention.
Number | Date | Country | Kind |
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0008536.5 | Apr 2000 | GB | national |
0018802.9 | Aug 2000 | GB | national |
This application is a continuation of U.S. application Ser. No. 10/240,796 filed May 30, 2003, which is a U.S. National Stage of International Application No. PCT/GB01/01246, filed Mar. 21, 2001, which itself claims priority to Great Britain Application No. 8536.5 filed Apr. 6, 2000, and Great Britain Application No. 18802.9, filed Aug. 2, 2000. The disclosures of the previous applications are hereby incorporated by reference herein.
Number | Date | Country | |
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Parent | 10240796 | May 2003 | US |
Child | 11330235 | Jan 2006 | US |