Optical: systems and elements – Optical modulator – Light wave temporal modulation
Patent
1998-03-11
1999-12-28
Ben, Loha
Optical: systems and elements
Optical modulator
Light wave temporal modulation
359245, 359248, 385 2, 385 8, 372 43, 372 50, G02B 2600
Patent
active
060089261
DESCRIPTION:
BRIEF SUMMARY
This invention relates to an optical modulator, and more particularly to a discrete modulator which modulates optical signals by electroabsorption.
Electroabsorption modulators (EAM) provide a convenient way of modulating optical signals, e.g. at wave lengths of 1300 nm and 1500 nm, in a wide range of fibre optical systems. Using sinusoidal voltage control for modulation they can generate short, e.g. having a duration of less than 10 ps, pulses. EAM are not only used to apply information carrying signals to unmodulated optical signals but they are also used for the rapid switching of optical signals, e.g. for demultiplexing of optical time division multiplexed signals.
It is important to distinguish between two forms of EAM, namely discrete and laser integrated EAM. A laser integrated EAM is a single device, grown as a single entity on the same chip, which includes a signal source (laser) and an EAM. The two functions are arranged so that the EAM modulates the signal produced by the source. In a discrete EAM the modulator is separate from the signal source, e.g. the signal is provided by fibre. It will be apparent that integrated EAM cannot be used for applications where the signal source must be separate, e.g. for demultiplexing or other applications where the signal comes from a remote source. Tanaka et al in the Journal of Lightwave Technology, volume 4, number 9 of September 1990 pages 1357-1362 describe integral EAM which include a thick layer of Fe-doped InP to provide electrical separation between the two functions of single chip. Koren et al in Appl. Phys Letters Vol 51, No. 15 dated Oct. 12, 1987 pages 1123 and 1124 describe discrete EAM with a large (4 .mu.m high and 6 .mu.m wide) absorber region which is therefore multimode with a low modulation depth with a slow switching speed.
It is emphasised that a discrete EAM is not integrated with the primary signal source but it may be associated or integrated with other devices.
EAM are semiconductor devices and they are usually implemented in III/V materials, for example materials containing at least one of indium, gallium or aluminum together with at least one of phosphorus and arsenic. Some EAM are buried heterostructure devices having a mesa or a ridge, usually formed of n-type material (such as n-doped indium phosphide), which extends into an electrical blocking region such as iron doped indium phosphide. There is an absorber region blocking region such as iron doped indium phosphide. There is an absorber region located on top of the mesa and this region provides a path for the optical signals to be modulated. In order to provide the guidance the absorber region has a higher refractive index than the material surrounding it. It will be appreciated that a waveguide comprises a path of higher refractive index surrounded by a cladding of lower refractive index. Thus the absorber region and its surroundings constitute a waveguiding structure. To achieve high modulation deaths it is appropriate that the absorber region support only a single optical mode. It there is more than one mode, i.e. if there are parasitic modes, then these will be transmitted in the surrounding material. In other words, if there are more than one modes only one (the principle mode) mode is transmitted in the absorber region.
The absorber region is located between the top of the mesa and a cap region, e.g. a cap region formed of p-doped indium phosphide. As is usual, a semiconductor device comprises a p-doped region and an n-doped region. The absorber region is located between these two regions. For example, the absorber region is often located in the depletion region of a pin-junction which is reverse biased. The absorber region may take the form of a single layer of semiconductor material with a uniform chemical composition having a suitable band gap. Preferably, it takes the form of a multi quantum well (MQW) system which consists of many, e.g. 19 to 49, interleaved layers of different semiconductor materials. For example the MQW may consist of n layers (usually called "wells
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Harlow Michael John
Moodie David Graham
Ben Loha
British Telecommunications public limited company
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