WO1997028584B1

WO1997028584B1 - Input and output monitored amplifier

Info

Publication number: WO1997028584B1
Application number: PCT/US1997/001593
Authority: WO
Filing date: 1997-01-30
Publication date: 1997-10-16

Abstract

A system and method are disclosed for monitoring input signal power for an optical amplifier without disturbing the input signal. In one embodiment, a small part of the output signal (114) is split off and then bandpass filtered to provide a bandpass signal with energy that is attributable only to amplified spontaneous radiation (ASE) generated within the optical amplifier. The power in the bandpass signal is measured by an optical detector (314). Then, using the measured bandpass signal power and a one-to-one correlation between bandpass signal power and input signal power, a controller (316) estimates the input signal power value that corresponds to the measured bandpass signal power.

Claims

-20-AMENDED CLAIMS[received by the International Bureau on 09 September 1 97 (09.09.97); original claims 1, 9 and 16 amended; remaining claims unchanged (4 pages)]

1. In an optical amplifier that includes at least one pump laser, a wavelength- division multiplexer ("WDM") and a section of active fiber, wherein said WDM forms a multiplexed signal by multiplexing a pump laser signal and an input light signal and wherein said active fiber, using power in said multiplexed signal contributed by said pump laser, generates an output signal from said amplifier that is amplified version of said input light signal, a system for input signal monitoring comprising: an output coupler connected to the output of said optical amplifier, said output coupler generating a split-off signal that is a lower power version of said output signal; an optical bandpass filter having a filter bandwidth lying within said amplifier's bandwidth and including a monitoring wavelength, said bandpass filter being configured to block light with power at a signal wavelength that is due to stimulated emission in said active fiber, said bandpass filter being coupled to said output coupler so as to provide a bandpass signal by performing bandpass filtering on said split-off signal; and an optical detector that generates a detector signal representing the power of light input to said optical detector, said optical detector being coupled to said bandpass filter so that said optical detector receives said bandpass signal, said optical detector thereby generating a detector signal that represents the power in said filter bandwidth of said split-off signal, values of said detector signal having a one-to-one correlation with input signal power values; said system for input signal monitoring excluding tapping of said input light signal so as to improve signal to noise ratio of signals at the output of said optical amplifier.

2. The system of claim 1 , wherein said power in said filter bandwidth is due to amplified spontaneous emission in said active fiber, a low detector signal value being correlated with a high input signal power and a high detector signal value being correlated with a low input signal power.

3. The system of claim 2, wherein said one-to-one correlation is described by an analytical function relating said detector signal and said input signal power, said system further comprising a microcontroller that computes the value of said input signal power by evaluating said analytical function at a particular value of said detector signal.

4. The system of claim 3, wherein said microcontroller is configured to issue an alarm when said input signal power is below an acceptable level.

5. The system of claim 2, wherein said one-to-one correlation is described by corresponding pairs of input and detector signal powers characterizing said optical amplifier, said system further comprising: a memory in which said corresponding pairs are stored; and a microcontroller having access to said memory that computes the value of said input signal power by interpolating from said corresponding pairs stored in said memory an input power value corresponding to a measured value of said detector signal.

6. The system of claim 1 , wherein said monitoring wavelength differs absolutely from said signal wavelength by at least said filter bandwidth.

7. The system of claim 6, wherein said filter bandwidth is substantially between 1510 nm and 1570 nm when said active fiber is an erbium-doped fiber amplifier.

8. The system of claim 1 , wherein said filter is a tunable filter whose passband is controllable.

9. In an optical amplifier that includes at least one pump laser, a wavelength- division multiplexer ("WDM") and a section of active fiber, wherein said WDM forms a multiplexed signal by multiplexing a pump laser signal and an input light signal and wherein said active fiber, using power in said multiplexed signal contributed by said pump laser, generates an output signal from said amplifier that is an amplified version of said input light signal, a system for input power monitoring comprising: an optical bandpass filter having a filter bandwidth lying within said amplifier's bandwidth and including a monitoring wavelength, said bandpass filter being configured to block light with power at a signal wavelength that is due to stimulated -22- emission in said active fiber, said bandpass filter being positioned in proximity to said active fiber so as to intercept radial emissions from said active fiber; and a wide-area optical detector that generates a detector signal that represents the power of light falling on said optical detector, said optical detector being configured in proximity to said bandpass filter so that said optical detector receives emissions from said active fiber that have all been filtered by said bandpass filter, said optical detector thereby generating a detector signal that corresponds to the power in said predetermined bandwidth of said radial emissions from said active fiber, values of said detector signal having a one-to-one correlation with input signal power values; said system for input signal monitoring excluding tapping of said input light signal so as to improve signal to noise ratio of signals at the output of said optical amplifier.

10. The system of claim 9, wherein said bandwidth power is due to amplified spontaneous emission in said active fiber, a low bandwidth power measurement being correlated with high input signal power and a high bandwidth power measurement being correlated with said low input power signal.

11. The system of claim 10, wherein said one-to-one correlation is described by an analytical function relating said detector signal and said input signal power, said system further comprising a microcontroller that computes the value of said input signal power by evaluating said analytical function at a particular value of said detector signal.

12. The system of claim 11 , wherein said microcontroller is configured to issue an alarm when said input signal power is below an acceptable level.

13. The system of claim 10, wherein said one-to-one correlation is described by corresponding pairs of input and detector signal powers characterizing said optical amplifier, said system further comprising: a memory in which said corresponding pairs are stored; and a microcontroller having access to said memory that computes the value of said input signal power by interpolating from said corresponding pairs stored in said -23- memory an input power value corresponding to a measured value of said detector signal.

14. The system of claim 9, wherein said monitoring wavelength differs absolutely from said signal wavelength by at least said filter bandwidth.

15. The system of claim 9, wherein said filter is a tunable filter whose passband is controllable.

16. A method for monitoring input signal power in an optical amplifier comprising the steps of: measuring output signal power of said optical amplifier at a monitoring wavelength that is substantially different from a signal wavelength associated with signals input to said optical amplifier; and based on a known, one-to-one correlation between said output signal power measured at said monitoring wavelength and said input signal power at said signal wavelength, estimating said input signal power at said signal wavelength, said method excluding measuring the input signal power at the input of said optical amplifier so as to improve signal to noise ratio of signals at the output of said optical amplifier.

17. The method of claim 16, wherein said step of measuring output signal power comprises: using an output coupler connected to output of said optical amplifier, generating a split-off signal that is a lower power version of said output signal, said output coupler reducing power in said output signal by an insignificant amount; using an optical bandpass filter having a filter bandwidth including said monitoring wavelength and excluding said signal wavelength, generating a bandpass signal by filtering said split-off signal; and using an optical detector that generates a detector signal that represents the power of light input to said optical detector, generating a STATEMENT UNDER ARTICLE 19

Differences between the referenced prior art (Bulow, Shimizu, Bayart and Masuda) and the present invention (referred to herein after as Jabr) are described herein.

1 ) All of Bulow, Shimizu, Bayart and Masuda require a coupler to extract light at the input end of the amplifier, thus going completely against the teaching of Jabr, which relies on eliminating the input coupler in order to improve the noise figure of the amplifier.

The table below summarizes these four prior art patents and compares to Jabr:

Requires input Requires Requires Purpose of Invention Extra coupler output coupler filter elements

Bulow yes no no measure input

Shimizu yes yes yes gain control feedback to control light

Bayart yes optional yes gain control feedback to pump lasers

Masuda yes yes yes measure noise heterodyne mixer

Jabr no yes yes monitor input without impacting the noise performance

2) In the background, p. 2 line 27-28, Jabr clearly states that the relation between input power and the output signal to noise ratio, hence the ASE (amplified spontaneous emission) is well known in the prior art. The novel or unobvious element in Jabr is the teaching of the location of the coupler at the output side of the amplifier, in contrast to the teachings of the above listed four patents.

3) The stated improvement of Jabr is a method for monitoring the input to an amplifier that minimizes the power loss at the input and the noise at the optical amplifier's output. -25-

4) While Bayart teaches the tapping of a portion of the light in the amplifier at the input side (see Fig. 2 of Bayart) for the purpose of controlling the amplifier gain via a servo loop, Bayart makes it optional (col. 3 line 61 ) to put the light tap at the output of the amplifier, thus failing to point out the importance of putting the tap specifically at the output in order to achieve better noise performance. According to Bayart therefore the location of the tap at the input or output is optional and in no way does Bayart suggest the importance of using an output tap.

5) Masuda's teaching requires both an input and output tap, again failing to point out the importance of avoiding the input tap altogether.

6) The combined failure of all four of Bulow, Bayart, Shimizu and Masuda, all of whom are presumed skilled in the art, to point out the importance of avoiding an input tap is clear evidence that argues against obviousness of the subject matter of this application.

7) It is known in the art to determine some signal attributes from ASE measurements. However, as the preceding discussions make clear, the prior art does not teach the avoidance of input taps.

Claims 1 and 16 are amended herein to clarify these distinctions between Jabr and the cited references. Specifically, the inserted language in claim 1 : "said system for input signal monitoring excluding tapping of said input light signal so as to improve signal to noise ratio of signals at the output of said optical amplifier," clearly shows that the claimed system excludes an input coupler. Claim 9 is amended in the same manner as claim 1. The inserted language in claim 16: "said method excluding measuring the input signal power at the input of said optical amplifier so as to improve signal to noise ratio of signals at the output of said optical amplifier," achieves a similar end for the claimed method. Both of these amendments are supported by the specification. For example, see page 9, lines 8-12.

For the reasons stated above, it is believed that claims 1 , 9 and 16 and their dependent claims are patentable over the cited art. -26-

It is also noted that cited references are inapplicable to claim 9 as none of the cited references employ an optical bandpass filter and a wide-area optical detector configured to generate a detector signal that can be used to determine input signal power based on power in a predetermined bandwidth that includes a monitoring wavelength and does not include the signal wavelength. For this additional reason claim 9 is believed to be patentable over the prior art.