Loop status monitor for determining the amplitude of the signal components of a multi-wavelength optical beam

A loop status monitor for determining the magnitude of the power output of each signal component of a multi-wavelength optical beam includes a dense wavelength division demultiplexing arrayed waveguide and a detector array. The detector elements are butt-coupled to each respective output of the waveguide to reduce insertion loss and cost. The detector array is formed of planar silica material having a plurality of precision ion implanted regions that separate the optical beam into its respective signal components. The signal components are directed to a corresponding detector element of the detector array. Each detector element includes a photodiode that converts the optical beam to an electrical signal indicative of the amplitude of the power of the channel.

FIELD OF THE INVENTION 
This invention relates to optical monitoring devices, and more particularly 
to a loop status monitor for demultiplexing a multi-wavelength optical 
beam into a plurality of component signals and determining the amplitude 
of the power of each component signal. 
CROSS REFERENCE TO RELATED APPLICATIONS 
Some of the matter contained herein is disclosed and claimed in the 
commonly owned U.S. patent application Ser. No. 08/885,428, entitled 
"Process For Fabrication And Independent Tuning Of Multiple Integrated 
Optical Directional Couplers On A Single Substrate" (Attorney Docket No. 
4827-09); U.S. patent application Ser. No. 08/885,449, entitled "Method 
and Apparatus For Dynamically Equalizing Gain In An Optical Network" 
(Attorney Docket No. 4827-11) and U.S. patent application Ser. No. 
08/884,747, entitled "Dynamic Optical Amplifier" (Attorney Docket No. 
4827-14) all of which are incorporated herein by reference. 
BACKGROUND OF THE INVENTION 
Wavelength division multiplexing (WDM) of optical beams is presently being 
used to increase the rate of transmission of information through an 
optical fiber. These multi-wavelength optical beams provide information by 
a plurality of signal components, also referred to as optical channels. 
Each channel is defined by a unique wavelength of light that are 
multiplexed together and transmitted through a communication link of an 
optical network. In order to detect any problems or potential problems of 
the optical network and to provide feedback to active components within 
the network, a plurality of status monitors may be located throughout the 
network to provide information indicative of the health or condition of 
the optical beams transiting through the network. This information 
includes the amplitude and signal to noise ratio of each of the channels 
of the optical beams. 
The status monitor also may be used to compensate for gain variations 
throughout the optical network. Current wavelength division multiplexed 
(WDM) intercity communications links, as shown in FIG. 1, require a number 
of amplifiers along the link length to compensate for fiber loss. The 
intercity links can run approximately 600 meters. There is currently a 
need for an amplifier every 80 to 120 kilometers in these links. 
Dynamic operating conditions, such as the addition and subtraction of the 
channels, and nonuniform gain characteristics of the amplifiers result in 
gain variations between the channels each time the optical beam is 
amplified. The prior art of FIG. 1 does not provide compensation for these 
conditions. 
FIG. 1 shows a graphical representation of a typical communication link 2 
of an optical network. A plurality of light generators (LG) 3 provide 
respective component signals 12 of select wavelengths that are combined by 
a multiplexer 4 to produce the multi-wavelength optical beam 14. Before 
the component signals are multiplexed, a plurality of pre-emphasis devices 
(PE) 5 amplify selectively each of the respective component signals 12. As 
mention hereinbefore, a plurality of amplifiers 20 amplify the optical 
beam 14 to compensate for fiber loss as the beam passes therethrough. At 
the receiver end of the communication link, the signal components 12 of 
the optical beam are then separated by a demultiplexer 6 and provided to a 
corresponding receiver (R) 7. 
The prior art 2 does not provide any compensation to overcome the 
nonuniform gain of the each amplifier 20. As shown in each of the plots 
14, which are representative of the amplitude of the output power of each 
channel 12, the differential of the output power of each of the channels 
increase after each gain stage 20. The output power of each channel of the 
communication link at 8, therefore, are not equal. The only compensation 
provided by the prior art is adjustment of the pre-emphasis devices 5 for 
amplifying each channel 12 to ensure adequate signal level and 
signal-to-noise is achieved. 
A device that provides feedback of the channels is a Fabry-Perot spectrum 
analyzer. This spectrum analyzer includes two pieces of an optical fiber 
that are coated to become a broadband high reflector and are 
laterally-spaced and aligned with each other. The distance between the 
ends of the fibers are varied to determined the power spectrum of each 
optical signal. The spectrum analyzer and associated electronic control is 
very costly. These devices are bulky and unreliable due to its moving 
parts. In addition, calibration of these devices are difficult to maintain 
due to drift and hysterisis. These spectrum analyzers also do not provide 
simultaneous monitoring of each channel, but scans through each channel of 
the multi-wavelength optical beam to provide a serial output. 
Accordingly, it is the principal object of this invention to provide an 
inexpensive, compact status monitor that provides signals indicative of 
the condition of each channel of an optical beam. 
It is another object of this invention to provide a status monitor that 
does not require calibration which permits the status monitor to be 
interchangeable throughout the optical network. 
It is a further object of this invention to provide a status monitor that 
generates simultaneously output signals of each channel that are 
indicative of the condition of each signal. 
SUMMARY OF THE INVENTION 
According to a preferred embodiment of the present invention, a loop status 
monitor is used in an optical network having a multi-wavelength optical 
beam with a plurality of component signals in which each component signal 
has an amplitude and an unique wavelength. The loop status monitor 
includes an optical waveguide for receiving and substantially 
simultaneously demultiplexing the multi-wavelength optical beam into a 
plurality of component signal beams. The loop status monitor also includes 
a detector array having a plurality of photodetectors for receiving said 
component signal beams. The detector array also provides substantially 
simultaneously a corresponding electrical feedback signal for each of the 
component signals indicative of the magnitude of the output power of the 
component signal. 
According to another embodiment of the present invention, a loop status 
monitor is used in an optical network having a multi-wavelength optical 
beam with a plurality of component signals in which each component signal 
has an amplitude and a unique wavelength. The loop status monitor includes 
an optical waveguide for receiving and substantially simultaneously 
demultiplexing the multi-wavelength optical beam into a plurality of 
component signal beams. The loop status monitor also includes a detector 
array having a plurality of photodetectors for receiving said component 
signal beams. The detector array also provides substantially 
simultaneously a corresponding electrical feedback signal for each of the 
component signals indicative of the magnitude of the output power of the 
component signal. The loop status monitor further includes a controller 
for receiving the corresponding electrical feedback signal for each of the 
component signals and generating a corresponding control signal for each 
of the component signals. The control signals are provided to a gain 
equalization module to remove the differences between the amplitudes of 
the output power of each of the component signals. 
According to another embodiment of the present invention, a method for use 
of generating a control signal for each component signal of a 
multi-wavelength optical beam of the amount of attenuation required for 
equalizing each component signal includes a step of demultiplexing the 
multi-wavelength optical beam into the corresponding plurality of 
component signals. The magnitude of the power of each of the component 
signals is then determined. The magnitude of the power of the component 
signals are compared; and a control signal. A control signal for 
presentation to a gain equalization module to remove differences between 
the amplitudes of the output power of each of the component signals is 
then generated. 
The above and other objects and advantages of this invention will become 
more readily apparent when the following description is read in 
conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 2, a block diagram of a preferred embodiment of a loop 
status monitor 10 for determining the condition of each channel 12 of a 
wavelength division multiplexed optical beam 14. The loop status monitor 
10 includes a dense wavelength division demultiplexing arrayed waveguide 
16 and a detector array 18 for separating the optical beam into its 
respective channel and determining at least the amplitude of each channel. 
The detector array 18 may also provide signals indicative of the condition 
of the channels, such as the signal to noise ratio. The loop status 
monitor serves the function of a low cost optical spectrum analyzer. 
The planar array waveguides 16 are currently made using silica on silicon 
technology. These are silica on silicon waveguides and are commercially 
available for multiplexing/demultiplexing in WDM networks. The planar 
waveguide includes a plurality of precision ion implanted regions for 
separating each channel 12 of the optical beam that direct the channels to 
a corresponding detector element 20 of the detector array 18. Each 
detector element 20 includes a photodiode that converts the optical beam 
to an electrical signal indicative of the amplitude of the power of the 
channel. In an alternative for the waveguide, any passive 
multiplexing/demultiplexing dielectric filter based device or Fiber Bragg 
Grating based device could be used with appropriate modifications. 
WDM 1.times.N arrayed demultiplexer waveguides are presently commercially 
available. These commercial waveguides include multiple fiber pigtail 
outputs that result in relatively high excess losses (approximately 7 dB). 
In contrast, the waveguide 16 embodied in the present invention is 
butt-coupled to each of the elements 20 of the detector array 18, and thus 
eliminates the output pigtails and reduces the associated insertion loss. 
One benefit of the loop status monitor 10 is that it provides greater than 
15 dB of channel isolation without additional filtering. If, however, 
there is too much cross-talk between the channels, Bragg gratings 22 may 
be written directly into the silica of the waveguide to increase the 
channel isolation and enhance the resolution in the monitor 10. Increased 
channel isolation also permits blocking of the WDM signals by tuning the 
grating, such as by heating the gratings, which permits detection of the 
amplifier noise level. This information will allow the signal-to-noise 
ratio to be computed. 
For use in a dynamic optical amplifier 24 as shown in FIG. 3, the output 
signals of each detector element 20 are provided to a controller 26. In 
response to these output signals, the controller 26 generates 
corresponding control signals representative of the degree of attenuation 
required to equalize the output signal of each channel of the amplifier 
24. Each respective control signal, therefore, is indicative of the 
difference of the amplitude of the respective channel 12 and the channel 
having the least amplitude. The control signals are provided to a gain 
equalization module 28 that will be described in greater detail 
hereinafter. 
In an alternative embodiment shown in FIG. 8, an AOTF 60, configured as a 
spectrum analyzer, along with appropriate control and feedback electronics 
can be used as a loop status monitor 10. In this embodiment, the AOTF 
scans through each network channel 12 of the multi-wavelength optical beam 
14, sequentially, monitoring the power in each channel. The selection of 
wavelengths can be changed in approximately 10 microseconds making the 
spectrum analyzer reconfiguration virtually instantaneous. From this data 
a channel intensity profile can be constructed and fed to appropriate 
network elements including the gain equalization module 28 (see FIG. 7), 
programmable add-drop modules and network health monitoring systems. 
The control and feedback electronics for the AOTF 60 include an RF signal 
generator 62 and a controller 26. The AOTF is adapted to receive a 
multi-wavelength optical beam and provide an output optical signal of a 
selected wavelength in response to an RF signal provided by the RF signal 
generator 62. The selected optical wavelength is a function of the 
wavelength of the RF signal provided to the AOTF 60. The controller 26 
provides a control signal that commands the RF signal generator to 
sequentially adjust the wavelength of the RF signal. Each RF signal 
provides a selected optical wavelength at the output of the AOTF. By 
serially adjusting the RF signal, the AOTF passes sequentially each 
optical wavelength 12 of the optical beam 14 to a detector element 20. The 
detector element then converts each optical wavelength to an electrical 
signal indicative of the amplitude of the power of each channel. The 
output signal can then be provided to a network controller that monitors 
the optical network or a dynamic optical amplifier. 
As described hereinbefore, the loop status monitor 10 may also be used to 
monitor the "health" and status, such as power and signal-to-noise ratio 
of each channel, at various locations within the network. 
Referring to FIG. 3, a block diagram of a high performance optical 
amplifier 24 with the required sophistication to handle reconfigurable 
networks provides a substantially uniform gain between each of the 
channels 12 of a dense wavelength division multiplexed optical beam 14 
propagating through an optical fiber 30 of an optical network. The 
amplifier 24 includes an optical isolator 32 that permits the optical beam 
to pass through and prevent optical noise from propagating back through 
the optical fiber. The optical amplifier further includes a gain stage (G) 
34, a gain flattening module (GFM) 36 and a gain equalization module (GEM) 
28 connected in series. The optical beam is amplified initially by the 
gain stage 34, wherein the gain of each wavelength is not uniform. The 
amplified signal then passes through the gain flattening module 36 and the 
gain equalization module 28. The gain flattening module selectively 
attenuates each of the channels 12 to compensate for the fixed nonuniform 
gain characteristics of the gain stage 34 which will be described in 
greater detail hereinafter. 
The gain equalization module 28 attenuates dynamically each of the channels 
in response to the control signals provided by the loop status monitor 10 
to equalize the amplitude of the power of each of the channels 12. As 
described hereinbefore, the loop status monitor 10 generates corresponding 
control signals provided to the gain equalization module 28. Each of the 
control signals is representative of the degree of attenuation required to 
equalize the output signal of the amplifier. 
The gain stage 34 includes an optical fiber doped with a rare earth ion, 
such as erbium and praseodymium. The gain as a function of color, or gain 
spectrum, of these doped optical fibers is not uniform, and is also 
dependent on the input power, the spectrum of the optical beam and the 
composition of the fiber. In the case of erbium which provides a gain in 
the 1500 nm-1650 nm communications window, as shown in FIGS. 4 and 5, the 
gain can vary from 3 to 10 dB depending upon the glass composition of the 
fiber. FIG. 4 shows a curve 36 of the gain spectrum of an optical signal 
passing through an erbium doped fluoride host glass fiber. FIG. 5 shows a 
curve 38 of the gain spectrum of an optical signal passing through an 
erbium doped silica host glass fiber. 
Referring to FIG. 6, the gain flattening module 36 is adapted to compensate 
for the nominal variation of the gain spectrum of the gain stage 34. The 
gain flattening module flattens the gain or, in other words, selectively 
attenuates the gain spectrum so that the gain differential between each 
channel is preferably approximately 0.1-0.5 dB. The selective attenuation 
by the gain flattening module 12 may be provided by dielectric filters or 
fiber gratings, such as Long Period Gratings as shown in U.S. Pat. No. 
5,430,817. 
FIG. 6 includes the pair of curves 36,38 showing the amplified amplitude of 
the optical beam from gain stage 34 as a function of wavelength. The gain 
stage 34 of curve 36 comprises an erbium doped fluoride host glass fiber 
and the gain stage of curve 38 comprises an erbium doped silica host glass 
fiber. The attenuation of the amplified signal by the gain flattening 
module is graphically shown in curve 40. A solid curve 41 represents the 
attenuation of the input optical beam as a function of the wavelength for 
silica host glass fiber and a dotted curve 43 represents the attenuation 
of the input optical beam as a function of the wavelength for fluoride 
host glass fiber. The attenuation for each doped fiber is proportionally 
inverse to its respective gain spectrum 36,38. The resulting gain spectrum 
from each respective doped fiber and gain flattening module is 
substantially equal between each channel, as shown in curve 42. 
Depending upon the spectral flatness of the passive fiber amplifier gain 
stage 20 (i.e. erbium doped fluoride host glass fiber), the passive gain 
flattening module 22 may not be required. This is true in the case where 
the gain equalization module 24 has sufficient bandwidth and dynamic range 
to provide both the gain flattening function and the gain equalization 
function simultaneously. 
Referring again to FIG. 3, the flattened optical signal from the gain 
flattening module 36 then propagates to the gain equalization module 28 
that substantially equalizes dynamically the amplitude of each channel 12. 
Dynamic gain equalization is necessary to compensate for the dynamic gain 
changes of the gain stage 34, such as gain tilt, gain ripple, hole 
burning, transient gain fluctuations, and any gain fluctuations from the 
gain flattening module. As described earlier, gain tilt is a function of 
the input power and spectra of the transmitted channels. As channels are 
added and subtracted from the optical beam, the input power changes and 
effectively "tilt" the gain of the amplifier in dependence of the 
wavelength of the channels. To compensate for gain tilt, the gain 
equalization module 28 selectively attenuates each respective channel in 
accordance to control signals provided by the loop status monitor 10. 
As shown in FIG. 7, the gain equalization module 28 incorporates an array 
of wavelength tunable fiber Bragg gratings 50 to attenuate each channel 
12. Each grating 50 is nominally aligned to a channel such that the 
transmission through the grating is varied by tuning the Bragg wavelength 
of the grating. The loop status monitor 10 measures the amplitude or power 
of each channel and provides a control signal back to each respective 
tunable grating 50 of the gain equalization module 28. Each grating 50 
then attenuates each channel 12 in response to the corresponding control 
signal to vary the amplitude of each transmission through the grating, so 
that the amplitude of each channel is substantially equal at the 
amplifier's output terminal 52. One skilled in the art would recognize 
that other methods of tuning, such as electro-optic and acousto-optic 
tunable filters (AOTF), may be used to selectively attenuate the channels 
of the optical signal. A preferred equalization module 28 is described in 
greater detail in Applicants' co-pending application for "Dynamic Gain 
Equalization Module", attorney docket no. 4827-11, which is incorporated 
herein by reference. 
Although the invention has been shown and described with respect to an 
exemplary embodiment thereof, it should be understood by those skilled in 
the art that the foregoing and various other changes, omissions, and 
additions in the form and detail thereof may be made therein without 
departing from the spirit and scope of the invention.