Optical power measurement in photonic integrated devices

Electricity: measuring and testing – Testing potential in specific environment

Reexamination Certificate

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C324S096000

Reexamination Certificate

active

06614213

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to photonic integrated devices, also referred to as photonic integrated circuits or optoelectronic integrated circuits, and is more especially concerned with measuring optical power in such devices.
A photonic integrated device produces a discernible change in a characteristic of light generated by it, passing through it or absorbed by it. Such a circuit may convert electricity into light or vice versa such as for example in an optical transmitter or receiver module for use in an optical telecommunications system. Alternatively it may change the power level of the light such as in an optical modulator or a splitter or a coupler, whether tunable or otherwise. The device may change a characteristic particular to the light such as its phase or frequency. A photonics integrated device can comprise a single device which is constructed in a monolithic form or comprise two or more such devices which are monolithically integrated together, giving rise to the term photonics integrated circuit. In particular photonics integrated devices may integrate, for example, interferometers, photodiodes, phase shifters, modulators, couplings, gratings and filters.
Optical power monitoring of photonic integrated circuits during their operation is essential in maintaining optimum performance. In optical telecommunications light power levels of several mW are typical though they can be up to several hundred mW in the case of pump lasers for fibre amplifiers. It is known in packaged laser modules to use a back-facet PIN photodiode to measure the optical power and this is usually integrated using hybrid techniques. However, for photonic integrated devices which contain only non-absorbing waveguides, such as electro-optic modulators, it is difficult to monolithically integrate photodiodes since the latter are based on materials having a high linear absorption. To integrate such devices requires regrowth techniques to form an absorbing PIN photodiode. Such techniques are time consuming and costly. Furthermore, since conventional photodiodes absorb nearly 100% of light incident on them, they must not be placed directly in the path of the optical power travelling through the waveguides. Therefore optical splitters are used to tap-off a certain fraction of the optical power from the waveguides. This further adds to the complexity and cost of fabricating the device.
In IEEE Photonics Technology Letters Volume 9 No. 4, April 1997, pages 493-495 there is disclosed an optical detector which comprises a GaAs ridge waveguide which measures optical power by measuring a two photon absorption photocurrent in the waveguide.
SUMMARY OF THE INVENTION
A need exists therefore for a method or an apparatus for measuring optical power in a photonics device which at least in part overcomes the limitations of the known arrangements.
According to the invention there is provided a photonic integrated device having a waveguide of semiconductor material which is monolithically integrated into the device and which has a low linear absorption at an intended wavelength of operation of the device, said device being arranged to perform a discernible change in a characteristic of light generated by it, passing through it or absorbed by it and characterized by an optical power detector which comprises means associated with said waveguide for measuring a photocurrent generated in the waveguide by two photon absorption without substantially affecting the light passing along the waveguide.
Two photon absorption (TPA) is a non-resonant, non-linear optical process which occurs in semiconductor materials for photons with energy less than the semiconductor band-gap E
g
, but greater than E
g
/2. The process occurs when an electron is excited from the valence band to an intermediate virtual state between the valence band and the conduction band by absorbing a first photon and is excited from the intermediate virtual state to the conduction band by absorbing a second photon. The intermediate virtual state can be any state in any band, although the transition probability is highest when the energy difference between the states involved is smallest, that when the intermediate state lies closest to the upper valence band or lower conduction band. A particular advantage of using TPA is that since only a small faction of the light is absorbed this eliminates the need for an optical splitter and a separate detector. Furthermore the detector can be readily monolithically integrated into devices having a waveguide of semiconductor material which is non-absorbing. Generally TPA is considered a parasitic effect though it has been proposed to use TPA in autocorrelators to measure pulse widths such as described in “The Two-Photon Absorption Semiconductor Waveguide Autocorrelator”, IEEE Journal of Quantum Electronics, volume 30, number 3 and “Autocorrelation Measurements of Modelocked Nd:YLF Laser Pulses Using Two-Photon Absorption Waveguide Autocorrelator”, IEEE Photonics Technology Letters, volume 9, number 5.
Preferably the waveguide transmits all of the light within the device rather than being branch thereof. That is, the detector is preferably an in-line device which detects light passing through the waveguide on its way to be used in a subsequent operation. The detector therefore acts as a power tap which taps-off a small proportion of light travelling through the waveguide. By “small amount” it is meant less than 20%. It may tap-off as little as less than 10%, 5% or even 1% depending on the waveguide material, the wavelength and optical power of the light.
The detector may be used to detect light in the range 200 nm to 4000 nm, and especially in the sub-range 800 nm to 1600 nm. Most preferably it is used to detect light in the regions of 1300 nm and 1550 nm.
Most preferably the means comprises one or more electrodes provided on the waveguide. Advantageously the or each electrode forms a Schottky contact with the waveguide material. A particular advantage of using electrodes which form a Schottky contact is that electrical isolation between the electrodes and/or any other electrodes which may be present in the device can be readily achieved by selective removal of the electrode material thereby eliminating the need for selective etching of the waveguide as would be required for an Ohmic contact. Although a Schottky contact is not able to withstand high currents it is ideally suited for measuring the relatively small TPA current.
Most preferably the or each electrode comprises metallic aluminum. In a preferred fabrication of the device the semiconductor waveguide material comprises gallium arsenide and gallium aluminum arsenide.
The device can have one or more light inputs and one or more light outputs and can be selected from a group of devices comprising an interferometer, a photodiode, a phase shifter, a modulator, a coupler, a grating, a filter, a beam former, a laser, an optical add-drop multiplexer or an optical cross connection. Most preferably the device comprises a Mach Zehnder type optical modulator.
According to a second aspect of the invention there is a method of measuring optical power in a photonics integrated device of a type having a waveguide of semiconductor material which is monolithically integrated into the device and which has a low linear absorption at an intended wavelength of operation of the device, said device being arranged to perform a discernible change in a characteristic of light generated by it, passing through it or absorbed by it, the method characterised by measuring a photocurrent generated in the waveguide by two photon absorption.
Advantageously the measured optical power is used to control or monitor the operation of the device. The measured optical power can for example be used to control the operation of the device by using a feedback arrangement. Alternatively the measured optical power can be used to detect data signals present in the light and change the operation of the device in response to the data signals.


REFERENCES:
patent: 4369363 (1983-01-01), Qu

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