Coherent light generators – Optical fiber laser
Reexamination Certificate
1997-08-19
2001-01-09
Scott, Jr., Leon (Department: 2881)
Coherent light generators
Optical fiber laser
C372S102000
Reexamination Certificate
active
06172995
ABSTRACT:
FIELD OF THE INVENTION
This invention is in the field of fiber optics, particularly in the field of optical amplifiers.
BACKGROUND TO THE INVENTION
Optical amplifiers, especially erbium doped fiber amplifiers (EDFAs), are largely used in the optical communication systems for increasing the signal transmission distance. It is well known that optical amplifiers, fiber or semiconductor based, with or without the presence of an input signal, emit a fluorescent light called Amplified Spontaneous Emission (ASE). This ASE is of low temporal coherence, and emits within a wide wavelength range.
In the case of an erbium doped fiber amplifier, ASE has a peak around 1530 nm, then the power density decreases, having a dip around 1542 nm, increases slightly again and reaches a plateau centered around 1550 nm. The spectrum has a 3-dB bandwidth of about 9 nm. The density difference between the peak at 1530 nm and the plateau around 1550 nm is about 15 dB. The spectrum is not flat.
A much wider 3-dB bandwidth and a flat spectrum output would be of great use for passive component testing, spectroscopy, fiber sensor, fiber gyroscopes, optical coherence tomography etc., for replacement of existing tunable lasers, for much lower cost and for much faster and easier testing results, especially for passive component manufacturers. Combined with a scanning filter, it would be able to provide a true continuously tunable light source without any frequency steps, which is critical for material structure analysis. However, the existing narrow 3 dB bandwidth is not sufficient for these applications.
SUMMARY OF THE INVENTION
The present invention provides a high power broadband optical source having a high long-term stability and a 3-dB bandwidth of more than 33 nm. It offers a high output power with a pumping efficiency of greater than 25%. Thus the signal produced using the apparatus of the present invention can be used to for the aforenoted applications.
In accordance with an embodiment of this invention, an optical amplifier medium is used as a fluorescent light emitting material. In the case of erbium fiber, the gain medium is pumped by a 980 nm or a 1480 nm laser diode. Amplified Spontaneous Emission (ASE) is generated and propagates in two directions: one is forward propagating ASE (i.e., in the same direction of transmission signal when it's used as an amplifier), the other one is backward propagating ASE (i.e., reversibly propagating compared to the usual signal direction). The natural ASE spectrum of the erbium fiber has huge power density difference between the section around 1530 nm and that around 1550 nm; usually it is about 15 dB or higher.
The amplification medium, in our case the erbium doped fiber has two sections. The first one, pumped by a small portion of the pump power (30% in a successful laboratory prototype), is used to generate a seed stage. The backward propagating ASE is used as a seed signal and is sent back into the gain medium through a 50% bi-directional coupler. Along with a fiber Bragg grating having characteristics as described below and positioned at the end of the seed stage, it provides a strong and broadband spectrum without the huge peak at 1530 nm.
The second stage, pumped by 70% of the pump power, comprises an amplification stage. When the back-feed seed signal enters into this stage, with a power of several mW, the gain medium is forced to work in deep saturation and thus emits a strong and equalized power density at each wavelength within the erbium emission window. The low plateau section of the ASE spectrum is enhanced. It gives a flat spectrum within 1.5 dB peak to peak difference. The 3-dB bandwidth becomes about 33 nm instead of 9 nm. The self-saturation results also in a much higher pump efficiency, 25 mW out of 100 mW pumping power, compared to a pure ASE source which has only about 10% pumping efficiency.
In accordance with an embodiment of the invention, a broadband optical source comprises (a) a first stage amplifier for producing an amplified spontaneous emission (ASE) optical seed signal having a forward propagating signal and a backward propagating optical signal, (b) feedback apparatus for feeding back the backward propagating signal to the first stage amplifier to generate a forward propagating broadband optical signal, (c) a filter for receiving the broadband optical signal, the filter having a transfer characteristic inverse to at least a portion of the broadband optical signal for flattening the broadband optical signal, (d) a second stage amplifier for amplifying the flattened broadband optical signal, and (e) apparatus for outputting the flattened broadband optical signal.
In accordance with another embodiment, a method of producing a broadband optical signal comprises (a) pumping a first stage amplifier with an optical pumping signal, to produce an amplified spontaneous emission (ASE) seed signal having a forward propagating optical signal and a backward propagating optical signal, (b) feeding back the backward propagating optical signal to the first stage amplifier to generate a forward propagating broadband optical signal, (c) passing the broadband signal through a filter having a transfer characteristic which is inverse to at least a portion of the broadband optical signal to flatten the broadband signal, and (d) amplifying and outputting the flattened broadband signal.
In accordance with another embodiment, a method of producing a broadband optical signal comprises generating an amplified spontaneous emission (ASE) signal in an optical amplifier having forward propagating and backward propagating portions thereof, feeding back the backward propagating portion to the optical amplifier to saturate it, and outputting a broadband optical signal from the optical amplifier.
REFERENCES:
patent: 5239607 (1993-08-01), da Silva et al.
JDS Uniphase Inc.
Jr. Leon Scott
Teitelbaum Neil
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