GB2385143A

GB2385143A - Optical modulator control

Info

Publication number: GB2385143A
Application number: GB0201201A
Authority: GB
Inventors: Robert Griffin; Steven James Borley
Original assignee: Marconi Optical Components Ltd; Bookham Technology PLC
Current assignee: Lumentum Technology UK Ltd
Priority date: 2002-01-19
Filing date: 2002-01-19
Publication date: 2003-08-13
Also published as: WO2003062915A1; GB0201201D0

Abstract

A device for controlling a Mach-Zehnder interferometer modulator (MZM) includes a two photon absorber (TPA) detector at each output 10, 12 of the modulator. The TPA detector comprises an aluminium electrode forming a Schottky contact. The outputs M1, M2 of the detectors are fed to a multiplier 50 by way of a differential amplifier 40. A local oscillator 51 drives a multiplier 50 at a pilot frequency . The output of the multiplier 50 is passed to an integrator 53 via a low pass filter 52 and thence to the control electrodes of the MZM.

Description

i 2385 1 43

Modulator Arrangement The invention relates to a modulator arrangement for modulating an optical signal in an optical communications or data system.

In this specification the term "light" will be used in the sense that it is used generically

in optical systems to mean not just visible light but also electromagnetic radiation having a wavelength between 800 nanometres (nary) and 3000nm. Currently the principal optical communication wavelength bands are centred on 1300nm, 1550nm 10 (C-Band) and l590nm (L- Band), with the latter bands receiving the majority of attention for commercial exploitation.

Exemplary wavelength division multiplex (WDM) systems operating in the 1550nm C-Band optical fibre communication band are located in the infrared spectrum with 15 International Telecommunication Union (ITU) 200, 100 or 50 GHz channel spacing (the so called ITU Grid) spread between 191 THz and 197THz.

Currently, high-speed optical transmission mainly employs binary amplitude keying, using either non-return-to-zero (NRZ) or return-to-zero (RZ) signalling formats, in 20 which data is transmitted in the form of binary optical pulses, i.e. on or oú In high bit rate optical transmission networks (lOGb/s and above), a transmitter usually comprises a continuous wave (COO) laser together with an external intensity modulator. External modulation allows zero, or controlled chirp, of the optical signal, providing optimum performance for transmission over dispersive fibre.

In such high speed optical transmission systems, light from the CW laser is applied to an optical intensity modulator. In general, the most widely used modulator arrangements in such systems comprise Mach-Zehnder modulators (IBM). MZMs have an optical transmission versus drive voltage characteristic, which is cyclic and is 30 generally raised cosine in nature. The half period of the m's characteristic, which is measured in terms of a modulating drive voltage, is defined as Vn. MZMs for optical transmission applications are generally travellin wave modulators such as described in the paper "High Speed m - v Semiconductor Intensity Modulators" by

Robert G WaLker, IEEE Journal of Quantum Electronics, Vol. 27, No 3, March 1991, pp 654-667.

To achieve suitable performance from a MZM, the DC bias point must be set and 5 maintained over the operating lifetime of the device. One known method of controlling the bias point of a modulator, principally to control drift with lithium niabate modulators, consists of applying a pilot tone to the modulator driver gain control. A fraction of the optical output signal is then tapped off as a monitor signal and detected with a linear photodiode. The monitor signal is synchronously detected 10 at the pilot frequency and the resulting error signal fed back to control the bias point.

This approach provides a robust acquisition and control of the bias point.

fZMs are typically fabricated in either lithium niobate or GaAs/AIGaA s materials.

With current technology it is not possible to fabricate linear photodetectors 15 monolithically on the same substrate as Moms. Incorporating linear photodetectors requires a hybrid approach whereby a fraction of the output beam is tapped off and directed to a discrete linear photodiode which may be co-packaged with the MZM.

This approach leads to additional assembly steps and significant optical loss, to the detriment of the overall system performance.

The present invention seeks to provide a method of controlling a modulator in which power is detected using two photon absorbers.

According to the invention there is provided a modulator arrangement for modulating 25 an optical signal for use in an optical system comprising a laser for producing an optical signal of a selected wavelength, which signal is applied to a semiconductor intensity modulator adapted to modulate the optical signal in dependence on a respective drive voltage, which modulator has two outputs, wherein the output power of each output is monitored by a respective two photon absorption detector, the 30 detector signals then being used to drive a feedback arrangement to control electrodes of the modulator.

The use of a respective two photon absorption detector on each output of the modulator surprisingly overcomes the difficulties associated with a monitor signal obtained using a single two photon absorption detector.

5 Preferably, the feedback arrangement comprises an oscillator adapted to provide a pilot frequency to drive a multiplier which is also fed with a signal in dependence of the monitor detector signals, a low pass fitter and integrator connected to the output of the multiplier, and a modulation data signal amplifier, the gain of which is dependent on the oscillator output.

Preferably, the respective monitored outputs of the intensity modulator are fed to a differential amplifier, the output of the differential amplifier being fed to the feedback arrangement. 15 Preferably, the semiconductor material is a Group m-v material. Preferably, the semiconductor material is GaAs/AIGaAs. An advantage of using GaAS/AlGaAs materials for fabricating MZMs is that they permit the integration of two photon absorption detectors as described in GB2339278B, the contents of which are incorporated herein by way of reference.

Preferably the intensity modulator is a Mach-Zehnder modulator.

Preferably the electrode driven by the feedback arrangement controls the modulator bias point(s).

An exemplary embodiment will now be described in greater detail with reference to the drawings in which: Fig. 1 shows a control map for a linear photodiode; 30 Fig. 2 shows a control map for a two photon absorption detector; Fig. 3 shows schematically an intensity modulator arrangement according to the invention, and Fig. 4 shows a control map for a modulator arrangement according to the invention.

Figure 1 shows a contour plot of drive amplitude against bias for a linear photodiode, assuming ideal band limited NRZ data, which corresponds to known modulator control methods. The central point corresponds to a quadrature bias (50% crossover) 5 and Via drive amplitude. Applying a pilot tone and synchronously demodulating corresponds to a partial derivative of the control surface with respect to the dithered variable. Applying the pilot tone to the drive amplitude and feeding back the resulting error signal to the bias control port provides a robust control scheme. As the control map shows, the circuit locks unambiguously at quadrature, with identical loop gain 10 within reasonable bounds of drive amplitude.

Figure 2 shows a corresponding plot for a two photon absorber, which clearly exhibits more complex behaviour. Unlike the case for a linear photodiode, there is no scheme to provide simple quadrature bias control.

Figure 3 shows a schematic modulator arrangement implemented in GaAs/AIGaAs comprising typically a single wavelength laser 2, for example a distributed feedback (I)FB) semiconductor laser due to its stable optical output for a given wavelength, which is operated to produce an unmodulated optical output of a selected wavelength, 20 typically an ITU wavelength channel. Other laser sources are possible. For example, the laser source 2 may alternatively be a tunable laser operable, nonconcurrently, at a plurality of different wavelengths, such wavelengths typically being ITIJ wavelength channels. 25 Light from the laser 2, is applied to the intensity modulator 6. The intensity modulator 6 comprises a Mach-Zehnder electro-optic modulator (My. As is known MZ s are widely used as optical intensity modulators and have an optical transmission versus drive voltage characteristic, which is cyclic and is generally raised cosine in nature. The half period of the MZM's characteristic, which is 30 measured in terms of a drive voltage, is defined as Vn. Within the modulator arrangement of the present invention the MZM 6, will be required to operate as an optical intensity modulator.. To achieve this the MZM 6 is driven with a voltage signal which causes phase modulation of one or both arms of the Mach-ZeEnder

s structure due to the electro-optic effect. The two phase modulated optical signals when combined at 30 interfere to form an intensity modulation of the optical signal at the MZM outputs. The intensity modulation may be formed using a 2x2 multi-mode interferometer (Mall) as JO, which has two output arms 10,12, where one of the 5 output arms (12) outputs to an optical waveguide for onward transmission. Either output 10, or 12 may be used as the principal output. Outputs 10 and 12 are optically out of phase.

To provide an output digital signal with maximum optical power in a 'one' state and 10 minimum optical power in the 'zero' state, the MALI 6 is driven with a binary voltage signal with peak-peak voltage equal to Vat, and the DC bias voltage adjusted to give 50% data crossover so as to optinnse the extinction ratio (ratio of maximum optical power to minimum optical power) of the output signal. The DC bias voltage may be summed together with the data signal, or applied to a separate electrode of the Mach 15 Zehnder modulator.

To enable the extinction ratio of the output signal to be monitored output power detectors 20, 22 are provided on each output arm 10, 12 of the modulator 6. Each detector comprises a two photon absorber (TPA) detector to produce a monitor signal.

20 A particular advantage of using a TPA detector is that only a small fraction of light is absorbed and the monitor may be monolithically integrated with the modulator obviating the necessity for a separate splitter and photodetector, which would be used should the modulators be implemented using alternative materials such as lithium rnobate. The TPA detector comprises an alurniniurn electrode forming a Schottky 25 contact over the waveguide. Two photon absorption is a non-resonant, non-linear optical process which occurs in semiconductor materials for photons having an energy less than the semiconductor band gap. The process occurs when an electron is excited from the valence band to an intermediate virtual state in the band gap by absorbing a first photon and is excited to the conduction band by absorbing a second photon. This 30 generates a photocurrent, which is related to the optical power in the waveguide. The photo current can then be amplified as at 44, 42 to form signals M1, M2 respectively, for use in a feedback arrangement.

The invention overcomes the difficulties associated with a monitor signal using a single TPA by utilising both the outputs 10,12 from the two matched TPAs 20,22.

Consequential to the fabrication process the output arm TPAs 2O, 22 will have equivalent performance characteristics. The output M1, M2 of each TPA 20, 22 is fed 5 to a differential amplifier 40, which is part of a control circuit 40, 50, 51, 52, 53, 54, which circuit functions as a synchronous detector adapted to permit the optical phase to be controlled to within ±2O('r/90 radians).

In an implementation of the synchronous detector, the output of the differential 10 amplifier 40 is fed to a multiplier 50, which is driven at a pilot frequency provided by a local oscillator 51. The output of the multiplier 50 is then passed to an integrator 53 via a low pass filter 52. The output of the integrator is then fed to the control electrode(s) of the MZM.

15 The local oscillator 51 signal is also applied to the gain control of the driver amplifier, so as to provide a small modulation ofthe output amplitude.

The local oscillator 51 provides a pilot tone at a relatively low frequency well below the data bandwidth frequency. In a 20GHz system, for example, a suitable frequency 20 for the pilot tone would be of the order of lkHz. The dc output of the integrator 53, acts to provide feedback to a control electrode(s) to maintain the desired operating point of the control loop. In an ideal system, the do component would be zero. The bias of the MALI intensity modulator 6 acts to adjust itself to obtain the minimum do output from the respective integrator 53 of the control circuit.

Figure 4 shows the contour plot provided by M1-M2 at the differential amplifier 40, which is similar to that of the linear photodiode in Figure 1. The simple use of the output of the differential amplifier as an error signal for control of the modulators tends to lose phase lock quite easily, whereas the application of a pilot tone to the 30 drive amplitude provides a robust control.

Using M1-M2 as a control signal, the demodulated output of the synchronous demodulator 40,50,51,52,53,54 can be set to zero at 50% crossover. This produces a robust control signal which locks to 50% regardless of the exact dove level applied.

5 Alternative circuit realizations may be implemented within the spirit of the invention, which is herewith defined by the claims.

Claims

1. A modulator arrangement for modulating an optical signal for use in an optical system comprising a laser for producing an optical signal of a selected 5 wavelength, which signal is applied to a semiconductor intensity modulator adapted to modulate the optical signal in dependence on a respective drive voltage, which modulator has two outputs, wherein the output power of each output is monitored by a respective two photon absorption detector, the detector signals then being used to drive a feedback arrangement to control 10 electrodes ofthe modulator.

2. A modulator arrangement according to Claim 1, wherein the feedback arrangement comprises an oscillator adapted to provide a pilot frequency to drive a multiplier which is also fed with a signal in dependence of the monitor 15 detector signals, a low pass filter and integrator connected to the output of the multiplier, and a modulation data signal amplifier the gain of which is dependent on the oscillator output.

3. A modulator arrangement according to Claim 1 or Claim 2, wherein the 20 respective monitored outputs of the intensity modulator are fed to a differential amplifier, the output of the differential amplifier being fed to the feedback arrangement

4. A modulator arrangement according to any preceding clann, wherein the 25 semiconductor material is a Group m - v material.

5. A modulator arrangement according to Claim 4, wherein the semiconductor material is GaAs/AIGaAs.

30

6 A modulator arrangement according to Claun 1 to 5, wherein the modulator is a Mach-Zehnder modulator.

7. A modulator arrangement according to Claim 1 to 6 wherein the feedback arrangement controls the modulator bias point(s).

8. A modulator arrangement substantially as described herein, with reference to and as illustrated in the accompanying drawings.