Optical modulator with pre-determined frequency chirp

#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 fibre 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 fibre 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 fibre communications system.

#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:

#FIG. 1 is a schematic representation of a known Mach-Zehnder optical modulator in plan view;

#FIG. 2 is a schematic sectional end view of a known Mach-Zehnder optical modulator fabricated in GaAs/GaAlAs through the line ‘AA’ of FIG. 1;

#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 GaAs/AlGaAs. A sectional end view through the line ‘AA’ of FIG. 1 of such a modulator is shown in FIG. 2. The optical modulator 20 comprises in order an undoped (semi-insulating) Gallium Arsenide (GaAs) substrate 22, a conductive doped n-type Aluminium Gallium Arsenide (AlGaAs) layer 24, a further layer of undoped Gallium Arsenide 26, a further layer of undoped AlGaAs 28 and a metallic conductive layer 30. The GaAs layer 26 provides the optical waveguides medium with the refractive index contrast between the AlGaAs layers 24 and 28 and GaAs layer 26 providing vertical confinement thereby constraining light to propagate within the layer 26. The optical waveguide arms (4, 6 see FIG. 1) of the modulator are defined within the GaAs layer 26 which are selectively etched into the AlGaAs layer 28 two mesas (plateau region) 32, 34. The mesas 32, 34 provide an in-plane effective refractive-index contrast that confines the light to a region beneath the mesa. As shown in FIG. 2 light is confined to two parallel paths, the waveguide arms, which pass into the plane of the paper as illustrated and which are denoted by the broken line 36, 38. The metallic layer 30 is appropriately patterned to overlay the mesas 32, 34 and constitutes the respective modulation electrodes 40, 42 of each waveguide arm. The electrodes 40,42 run the length of the waveguide arms.

#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 metalisation layer 30, which form air bridges over the isolation trenches 46, 48 to respective modulation drive voltage lines 40b, 42b. As shown in FIG. 2 the left hand modulation drive voltage line 40b comprises an RF modulating drive line and the right hand line 42b the RF modulating drive voltage ground.

#Referring to FIG. 3 there is shown drive circuitry for operating the optical modulator of FIG. 2. To enable a dc bias potential to be applied to the backplane electrode 44 whilst still allowing the backplane to float at the RF modulation frequencies a dc-coupling capacitor Cd 50, inductor Ld 52 and drive resistor Rd 65 are connected as shown in the diagram. In practice the capacitor 50 is realised by a Schottky contact metalisation while the inductor Ld 52 and drive resistor Rd 65 are realised as narrow trench-isolated regions of the lead-in or lead-out waveguide runs which do not include modulating electrodes. As seen in FIG. 3 the modulating RF voltage Vmod is applied to the modulating electrodes 40, 42 in series whilst the bias voltage is applied in a parallel configuration. This drive arrangement ensures that the reverse bias conditions across the depletion layer (i.e. across layers 24, 26, 28) of the device are maintained throughout the cycle of the RF modulating voltage.