Polarization independent-type semiconductor optical amplifier

Optical: systems and elements – Optical amplifier – Particular active medium

Reexamination Certificate

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Reexamination Certificate

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06487007

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a polarization independent-type semiconductor optical amplifier, more specifically to a polarization independent-type optical amplifier for use in wavelength division multiplexing (WDM) communication systems, which has large fiber out saturation powers in a small size and at low consumed electric power.
Recently, corresponding to the drastic increase of communication demand, wavelength division multiplexing systems for multiplexing signal light of a plurality of different wavelengths and transmitting the signal light concurrently in one optical fiber are progressively developed. Such wavelength division multiplexing system includes a number of optical parts for combining, switching and dividing optical signals, and the optical signals are attenuated due to losses in the respective optical parts.
Optical amplifiers are used for compensating such attenuation. A very larger number of optical amplifiers are required in comparison with a number of optical amplifiers used in the conventional optical fiber systems. It is required that the optical amplifiers are small sized and operable at low power consumption.
In addition, it is required that such optical amplifiers used in in-line have less polarization dependence of gains because polarization states of input signal lights are random, and have large fiber out saturation powers so as to have wide input dynamic ranges because fluctuations of power levels of input signal light are large.
Among such various optical amplifiers, semiconductor optical amplifiers (SOAs) are small sized and have small power consumption, and can be designed to be polarization-independent. The semiconductor optical amplifiers are expected to be a loss compensating optical amplifier suitably used in the wavelength division multiplexing systems.
Such polarization independent-type semiconductor optical amplifiers for a 1.55 &mgr;m-band in the wavelength band used in the fiber optical communication have been conventionally developed. Such polarization independent-type semiconductor optical amplifier will be explained below.
An internal gain is a gain of the optical amplifier itself. A fiber to fiber gain is a gain of a system as a whole including an optical amplifier provided between an optical fiber on the input side and an optical fiber on the output side with an optical system for optical coupling, such as a lens, etc., disposed therebetween, which takes into account a loss of the system between the exit end face of the input side optical fiber and the incidence end face of the output side optical fiber.
A chip out saturation power is a chip out power given when an internal gain is decreased by 3 dB. A fiber out saturation power is a fiber output power given when a fiber to fiber gain is decreased by 3 dB.
As a semiconductor optical amplifier using a strain-free bulk active layer, P. Doussiere et al., Alcatel have realized a device which includes a bulk active layer of a 430 nm-thick and a 500 nm-width, and which has, at 800 &mgr;m-device length and at 200 mA injection current, a below 0.5 dB inter-polarization gain difference, a 29 dB fiber to fiber gain and a +9.0 dB
m
fiber out saturation power (see, e.g., P. Doussiere et al., IEEE Photon. Technol. Lett., vol. 6, pp. 170-172, 1994 and P. Doussiere et al, OAA '95, pp. 119-122).
As a semiconductor optical amplifier using a strained Multiple Quantum Well (MQW) active layer formed of a strain-free well layer and an tensile strained barrier layer, Magari et al., NTT realized a device including a strained MQW layer which is formed of 10 well layers each having a 5 nm-thick and a 0% strain amount and 11 barrier layers each having a 5 nm-thick and a −1.7% strain amount, and is sandwiched between 50 nm-thick and 100 nm-thick separate confinement heterostructure (SCH) layers, and which has, at a 660 &mgr;m-device length and 200 mA injection current, a below 1.0 dB inter-polarization gain difference, a 27 dB internal gain (a 13 dB fiber to fiber gain), a +14.0 dB
m
chip out saturation power (a +7.0 dB
m
fiber out saturation power) (see, e.g., K. Magari et al., IEEE Photon. Technol. Lett., vol. 2, pp. 556-558, 1990, K. Magari et al., IEEE Photon. Technol. Lett., vol. 3, pp. 998-1000, 1991, and K. Magari et al., IEEE J. Quantum Electron., vol. 30, pp. 695-702, 1994).
As a semiconductor optical amplifier similarly using a strained MQW active layer formed of a strain-free well layer and a tensile strained barrier layer, A. E. Kelly, et al., BT realized a device including a strained MQW layer which is formed of 10 well layers of a 0% strain amount and 11 barrier layers of a −0.67% strain amount and is sandwiched between 25 nm-thick SCH layers, and which has, at a 2000 &mgr;m-device length and 200 mA injection current, a below 0.5 dB inter-polarization gain difference, a 27 dB fiber to fiber gain and a +7.5 dB
m
fiber out saturation power (see, e.g., A. E. Kelly et al., Electron Lett., vol. 32, pp. 1835-1836, 1996 and A. E. Kelly et al., Electron Lett., vol. 33, pp. 536-538, 1997).
As a semiconductor optical amplifier using a strained MQW active layer formed of a compressive strained quantum well layer, a tensile strained quantum well layer and a strain-free barrier layer, M. A. Newkirk et al., ATT realized a device including a strained MQW layer which is formed of 3 compressive strained quantum well layers each having a 3.5 nm-thick and a +1.0% strain amount, 3 tensile strained quantum well layers each having a 16.0 nm-thick and a −1.0% strain amount, and 7 barrier layers each having a 10 nm-thick and a 0% strain amount, and which has, at a 625 &mgr;m-device length and 150 mA injection current, a below 1.0 dB inter-polarization gain difference, a 13 dB internal gain (a 4.4 dB fiber to fiber gain) and a +11.1 dB
m
chip out saturation power (a +6.8 dB
m
fiber out saturation power) (see, e.g., M. A. Newkirk et al., IEEE Photon. Technol. Lett., vol. 4, pp, 406-408, 1993).
As a semiconductor optical amplifier using a strained MQW active layer formed of a compressive strained quantum well layer and a tensile strained barrier layer, D. Sigogne et al., CNET realized a device which includes a strained MQW layer formed of 16 compressive strained quantum well layers each having a 8 nm-thick and a +1.1% strain amount and 16 tensile strained barrier layers of a 7 nm-thick and a −0.9% strain amount, and which has, at a 940 &mgr;m-device length and 150 mA injection current, a below 1.0 dB inter-polarization gain difference, a 23 dB fiber to fiber gain and a +7.0 dB
m
chip out saturation power (a +3.5 dB
m
fiber out saturation power) (see, e.g., A. Ougazzaden et al., Electron. Lett., vol. 31, pp. 1242-1244, 1955, D. Sigogne et al., ECOC95, pp. 267-270, and D. Sigogne et al., Electron. Lett., vol. 32, pp. 1403-1405, 1996).
As a semiconductor optical amplifier using a tensile strained bulk active layer, J. Y. Emery et al., Alcatel realized a device which includes a 200 nm-thick bulk active layer sandwiched on both sides thereof by 100 nm-separate confinement heterostructure layers and having a −0.15% tensile strain at a 1.2 &mgr;m-active layer width, and which has, at a 1000 &mgr;m-device length and 200 mA injection current, a below 0.3 dB inter-polarization gain difference, a 29 dB fiber to fiber gain and a +9.5 dB
m
fiber out saturation power (see, e.g., J. Y. Emery et al., ECCO96, vol. 3, pp. 165-168 and J. Y. Emery et al., Electron. Lett., vol. 33, pp. 1083-1084, 1997).
Polarization independent-type semiconductor optical amplifiers of various active layer structures as described above have been studied. In such semiconductor optical amplifiers, in order to obtain a wide dynamic range it is required that a fiber out saturation power, which provides an upper limit of the dynamic range, is as large as possible. For example, for a semiconductor optical amplifier for 1.55 &mgr;m-band having polarization dependency, the MQW active layer structure can provide a +19.5 dB
m
chip ou

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