TECHNICAL FIELD

The present invention relates to an optical spectrum analyzer (OSA) for measuring the optical power as a function of wavelength, and, more particularly, the spectral characteristics of optical channels in a wavelength division multiplexed (WDM) communication system.

BACKGROUND OF THE INVENTION

Current WDM communication systems strive for maximum transmission capacity by spacing optical channels as closely as possible, typically less than a nanometer (nm). As the channel spacing, however, decreases, monitoring the spectral characteristics of the channels becomes more critical in verifying system functionality, identifying performance drift, and isolating system faults. For example, such monitoring is critical in detecting wavelength drift, which can readily cause signals from one optical channel to cross into another. Also, with the recent use of optical amplifiers, real-time feedback to network elements has become increasing critical to ensure stable operation.

Optical instruments, called optical spectrum analyzers (OSAs), are known in the art, however, for measuring the optical power as a function of wavelength. Indeed, most conventional OSAs use a wavelength tunable optical filter, such as a Fabry-Perot interferometer or diffraction grating, to resolve the individual spectral components. In the latter case, light is reflected off the diffraction grating at an angle proportional to the wavelength. This is so, inasmuch as the grating lines cause the reflected rays to undergo constructive interference only in very specific directions. The spectrum of the light is then analyzed on the basis of the angle at which the light is diffracted using a detector array. Alternatively, the diffracted light is moved over a slit and then detected using a small detector.

Alternatively, a Fabry-Perot interferometer may be used consisting of two highly reflective, parallel mirrors that act as a resonant cavity, which transmits light only at a unique frequency (wavelength). Wavelength tuning may be accomplished by varying the mirror spacing or rotating the interferometer with respect to the incident light so as to provide an optical spectrum analysis.

Other OSAs known in the art are based on the Michelson interferometer, wherein the incident light is split into two paths. One path is fixed in length, and the other is variable so as to create an interference pattern between the signal and a delayed version of itself, known as an interferogram. The wavelength of the incident light can be determined by comparing the zero crossings in the interferogram with those for a known wavelength standard. The optical spectrum, however, is determined by performing a Fourier transform on the interferogram.

Although conventional OSAs perform acceptably, they are generally bulky as well as costly. Furthermore, it may take from a few seconds to a few minutes to obtain the optical spectrum, unsuited for next generation WDM communication systems that operate near or in excess of 10 Gbps, which require real-time monitoring.

Accordingly, it would be desirable to provide for a low cost, compact OSA capable of determining the spectral characteristics of the optical channels in real time.

SUMMARY OF THE INVENTION

The present invention is an optical spectrum analyzer (OSA) comprising a tree-structure of N-stage wavelength filters, a “wavelength slicer,” which “slices” the incident optical signal into desired groupings of individual “sliced” spectral components, each along a different output optical fiber. Cascaded fiber Bragg gratings and delay lines coupled to each output optical fiber then uniquely map the “sliced” spectral components into the time domain such that each spectral component is allocated a unique time slot.

In one embodiment, the optical spectrum analyzer comprises a three-stage, tree-structure of Mach-Zehnder interferometers which successively “slice” a WDM optical signal into eight (8) groupings of alternating spectral components. Each of the individual spectral components in each of the groupings is then allocated a unique time slot using two sets of delay lines, implemented preferably using fibers. A first set of delays delays each spectral component within each grouping of spectral components by a multiple integer time period. For each optical fiber, fiber Bragg gratings are also arranged such that the shortest wavelength spectral components are reflected first and the longest last. Then, second sets of delays disposed between each fiber Bragg grating, delay the reflections from each successive fiber Bragg grating so as to offset each new grouping of sliced spectral components uniformly in the time domain. In this latter manner, each sliced spectral component is thus allocated a unique slot within the time domain for conversion into an electrical signal by corresponding optical detectors.

In a second embodiment, a single stage is employed which, instead, “slices” the WDM optical signal into four consecutive bands of “sliced” spectral components. Similarly, each spectral component in each of the four bands of “sliced” spectral components is then allocated a unique time slot using two sets of delay lines. A first set of delays delays each band of spectral components by a multiple integer time period such that each of four bands is separated from one another in the time domain. Fiber Bragg gratings having corresponding transmission peaks are again arranged such that the shortest wavelength spectral components within each band are reflected first and the longest wavelength last. Additionally, second sets of delays delay the reflections from each successive fiber Bragg grating by an amount that offsets each desired sliced spectral component from one another in the time domain.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the present invention will become more readily apparent from the following detailed description of the invention in which like elements are labeled similarly and in which:

FIG. 1

is a generalized block diagram showing an optical spectrum analyzer in accordance with the principles of the invention;

FIG. 2

is a block diagram of the “wavelength slicing” stage of the optical spectrum analyzer of

FIG. 1

;

FIG. 3

is a graph illustrating the filter transmissions of the passbands of the “wavelength slicing” stage of

FIG. 2

;

FIG. 4

is a block diagram of the “time mapping” stage of the optical spectrum analyzer of

FIG. 1

;

FIG. 5

is an illustration of a fiber based Mach-Zehnder interferometer;

FIG. 6

is a graph of the intensity transmissions of the Mach-Zehnder interferometer of

FIG. 5

;

FIG. 7

is a block diagram of a first embodiment of an optical spectrum analyzer employing a three-stage “wavelength slicing” stage;

FIG. 8

is a graph of the sliced spectral components in the optical spectrum analyzer of

FIG. 7

;

FIG. 9

is a graph of sliced spectral components in the optical spectrum analyzer of

FIG. 7

as a function of time and wavelength, after propagating through a first set of fiber delay lines;

FIG. 10

is a graph of the sliced spectral components in the optical spectrum analyzer of

FIG. 7

as a function of time and wavelength, after propagating through a second set of fiber delay lines;

FIG. 11

is a graph of the intensity transmissions of the upper arms of the Mach-Zehnder interferometers in the optical analyzer spectrum of

FIG. 7

;

FIG. 12

is a block diagram of a second embodiment of an optical spectrum analyzer employing a single-stage “wavelength slicing” stage; and

FIG. 13

is a graph of the sliced spectral components in the optical spectrum analyzer of

FIG. 12

as a function of time and wavelength, after propagating through a first set of fiber delay lines.

DETAILED DESCRIPTION

The present invention is an optical spectrum analyzer (OSA) for measuring the power versus wavelength spectrum for an optical signal. It is based, in part, on the mapping of the spectral components of the incident optical signal into the time domain using a two-stage dispersive optical system, consisting preferably of all fiber based Mach-Zehnder interferometers, delay lines, and Bragg gratings, as discussed more fully herein below.

More specifically, a tree-structure of N-stage wavelength filters or “wavelength slicer” functions to “slice” the incident optical signal into desired groupings of individual “sliced” spectral components, each grouping transmitted along a different output optical fiber. Cascaded fiber Bragg gratings and delay lines coupled to each output optical fiber then uniquely map the sliced spectral components into the time domain. That is, each spectral component is allocated a unique time slot for conversion into an electrical signal by a corresponding optical detector. Although requiring post processing in the time domain, such a tree and pipe-line architecture can be readily fabricated with fibers, which reduces cost and complexity, among other things.

Without any loss of generality or applicability for the principles of the invention, the preferred embodiments are described with respect to measuring the spectral characteristics of optical wavelength channels in a WDM communication system. It should be clearly understood, however, that the present inventive OSA is capable of measuring the optical spectrum, in general, of a broadband light source.

Shown in

FIG. 1

is a generalized OSA

100

in accordance with the principles of the invention applied to measuring the spectral characteristics of a WDM signal

105

carrying k independent optical channels separated &Dgr;&lgr; apart from &lgr;

a

to &lgr;

b

. Each of the k optical channels is allocated a unique band of wavelengths of width up to &Dgr;&lgr;, over which information is transmitted using well-known modulation techniques. WDM signal

105

, which preferably occupies a total bandwidth of k&Dgr;&lgr; in the commonly used 1.528-1.560 &mgr;m “C-Band,” is to be resolved into 2

N

M individual spectral wavelength components, herein denoted as

∑

1

2

N

⁢

⁢

M

⁢

⁢

λ

i

=

λ

1

⁢

…

⁢

⁢

λ

2

N

⁢

M

,

with a corresponding resolution R of

k

⁢

⁢

Δ

⁢

⁢

λ

2

N

⁢

⁢

M

,

where N and M are integers. Note that &lgr;

i

herein refers to corresponding spectral components at intervals of

k

⁢

⁢

Δ

⁢

⁢

λ

2

N

⁢

⁢

M

from &lgr;

a

to &lgr;

b

.

WDM signal

105

is optically coupled to an N stage filter or a so-called “wavelength slicer”

110

for filtering or “slicing” into 2

N

desired groupings of individual sliced spectral wavelength components (&lgr;

1

)

1

. . . (&lgr;

1

)

M

through (&lgr;

2

N

)

1

. . . (&lgr;

2

N

)

M

, transmitted along 2

N

optical fibers

115

1

through

115

2

N

, respectively. Each 2

N

wavelength grouping consists of M individual “sliced” spectral components, spaced

k

⁢

⁢

Δ

⁢

⁢

λ

M

or every other 2

N

th

wavelength component, as discussed herein below. The actual sliced spectral components that appear on optical fibers

115

1

-

115

2

N

, however, are dependent on the number of stages, among other things.

Referring to

FIG. 2

, this so-called “wavelength slicing” function may be performed by utilizing 2

N

−1 filters

120

1

. . .

120

2

N

−1

configured preferably in a tree architecture. Each filter

120

1

. . .

120

2

N

−1

has an input fiber port A, and two output fiber ports B and C, each which exhibit periodic transmission passbands of a free spectral range (FSR) of

k

⁢

⁢

Δ

⁢

⁢

λ

2

N

-

n

⁢

⁢

M

(n=1, 2 . . . N), as depicted in

FIG. 3

, wherein n is the stage number, and N is the total number of stages. Importantly, note that the transmission passbands for ports A-B and A-C are offset by half the FSR, such that the centers of the passbands uniquely match corresponding “sliced” spectral components transmitted on ports B and C. In effect, this “slicing” optically samples WDM signal

105

with an overall resolution of the first-stage filter's FSR divided by two, wherein the sampled or sliced spectral components, appear alternating on the output ports.

Referring again to

FIG. 2

, for each successive stage, the transmission of sliced spectral components of WDM signal

105

inputted to fiber port A are alternated between the two output fiber ports B, C as the wavelength at port A changes. With each grouping of the so-called “sliced” wavelength components coupled into the input fiber port of a next-stage filter having twice the FSR as the previous stage, 2

N

groupings of “sliced” spectral wavelength components spaced

k

⁢

⁢

Δ

⁢

⁢

λ

M

appear at optical fibers

115

1

. . .

115

2

N

. As such, a different grouping of every 2

N

th

spectral component of WDM optical signal

105

is transmitted on each optical fiber

115

1

. . .

115

2

N

.

Referring back to

FIG. 1

, the present invention also utilizes a time slot allocation stage

120

so as to then uniquely map into the time domain the groupings of sliced spectral components (&lgr;

1

)

1

. . . (&lgr;

1

)

M

through (&lgr;

2

N

)

1

. . . (&lgr;

2

N

)

M

. As shown in

FIG. 4

, on the 2

N

optical fibers

115

1

. . .

115

2

N

, there are M cascaded fiber Bragg gratings (

125

1

)

1

. . . (

125

1

)

M

through (

125

2

N

)

1

. . . (

125

2

N

)

M

, respectively, each reflecting one of the corresponding M spectral components while transmitting all other unselected components. As such, the transmission peaks, i.e., Bragg wavelengths, of the fiber Bragg gratings corresponding to each optical fiber

115

1

. . .

115

2

N

are similarly spaced at intervals of

k

⁢

⁢

Δ

⁢

⁢

λ

M

from &lgr;

a

to &lgr;

b

.

Optical couplers or circulators

130

1

-

130

2

N

couple the corresponding “sliced” wavelength components (&lgr;

1

) . . . (&lgr;

1

)

M

through (&lgr;

2

N

)

1

. . . (&lgr;

2

N

)

M

into time slot allocation stage

120

, while allowing reflected spectral components from the fiber Bragg gratings to egress therefrom along optical fibers

135

1

-

135

2

N

.

Those skilled in the art will readily note that the fiber Bragg grating consists of a periodic variation in the refractive index of the corresponding fiber's core, which variation in the figures has been denoted by heavy marks. The fiber Bragg gratings may be fabricated by exposing the fiber to UV radiation through, for example, a single phase grating mask. Of course, chirped as well as apodized fiber Bragg gratings may be employed to vary the strength and length of the variations in the refractive index so as to remove unwanted sidelobes.

Importantly, fiber Bragg gratings (

125

2

)

1

. . . (

125

2

)

M

through (

125

2

N

)

1

. . . (

125

2

N

)

M

are arranged such that the shortest wavelength spectral components are reflected first and the longest wavelength components last. Fiber-based delay lines (

140

1

)

1

. . . (

140

1

)

M−1

through (

140

2

N

)

1

. . . (

140

2

N

)

M−1

, judiciously disposed, for example, between each fiber Bragg grating, delay the reflections from each successive grating such that each spectral component is correspondingly delayed so as to be uniformly separated in time, and in the order of its spectral wavelength number (i.e., &lgr;

1

<&lgr;

2

<. . . &lgr;

2

N

). In doing so, each spectral component is thus allocated a unique slot within the time domain for conversion into an electrical signal

145

by corresponding optical detectors

150

1

. . .

150

2

N

coupled to optical fibers

135

1

-

135

2

N

, respectively. Since the amplitude peaks of electrical signal

145

correspond to the optical power in the spectral components of WDM signal

105

, a display of the optical power as a function of wavelength can be readily obtained. This latter temporal mapping of the individual 2

N

M sliced spectral components of WDM signal

105

is shown in the depiction of electrical signal

145

of

FIGS. 1 and 4

.

Referring to

FIG. 5

, N-stage filter structure

110

is preferably constructed by utilizing unbalanced fiber-based Mach-Zehnder interferometers

155

. Mach-Zehnder interferometers

155

each consists of input and output couplers

160

,

165

, such as 50/50 splitters and the like, interconnected by two fibers

170

,

175

of unequal lengths d

1

, d

2

which introduce a desired phase difference between the signals. Note that the fibers between the couplers are referred to as interferometer arms.

The basic operating principle is that the incident optical signal P

O

is split into the two arms

170

,

175

of the interferometer. When the optical signals recombine, some or all of the optical power in one arm is transferred to the other because the signals interfere with each other, depending on the amount of phase difference between arms

170

,

175

. However, with the phase difference dependent on wavelength, such a device exhibits periodic passbands, wherein the intensity transmissions T

1

, T

2

are calculated to be in the main branch B

T

1

=

cos

2

⁢

⁢

(

Π

⁢

⁢

λ

FSR

)

and, in the secondary branch C,

T

2

=

sin

2

⁢

⁢

(

Π

⁢

⁢

λ

FSR

)

which are complementary, i.e., T

1

+T

2

=1.

As depicted in

FIG. 6

, the adjacent transmission peaks within the same arm of the interferometer are spaced apart by the free spectral range (FSR), and given by

FSR

=

λ

2

OPD

where OPD is the optical path difference between the arms of the interferometer.

Note that the Mach-Zehnder interferometer performs the above so-called “wavelength slicing” by transmitting on each output branch, alternate spectral components of the incident optical signal with respect to the input, which alternating spectral components are separated by FSR. With respect to a multiplexed optical signal, a WDM optical stream with channel spacing of &Dgr;&lgr; will be divided between the two outputs, such that alternate spectral components with a resolution FSR go to outputs B and C.

Although due to cost and packing considerations, the Mach-Zehnder interferometers are preferably implemented with discrete optical fibers, they may alternatively be implemented with an integrated optical device using planar waveguides. Of course, other suitable optical devices may be used to construct tree-structure filter

110

, such as WDM demultiplexers, routers or filters, using the acoustooptic optic or electro-absorption effects.

To more clearly illustrate the principles of the invention, shown in

FIG. 7

is a three-stage (N=3) OSA

100

for measuring, with a resolution R=0.04 nm, the optical spectrum of WDM signal

105

, consisting of optical channels spaced &Dgr;&lgr;=0.4 nm (50 GHz) within, for example, the so-called “C-band” (1528-1564 nm). To ensure that the spectral components in the time domain do not overlap, WDM signal

105

is pulsed, using for example an optical switch

180

, which samples WDM signal

105

for a duration less than the time domain channel spacing, such as 0.4 nsec.

The first stage consisting of fiber based Mach-Zehnder interferometer

155

1

has an input optical fiber A for receiving WDM signal

105

, and two output optical fibers, B

1

-B

2

. With FSR=2R=0.08 nm, Mach-Zehnder interferometer

155

1

slices WDM signal

105

into two groupings of alternating spectral components, with a first grouping

185

1

of odd spectral components (&lgr;

1

, &lgr;

3

. . . &lgr;

2

N

M−1

) communicated on optical fiber B

1

, and a second band

185

2

of even spectral components (&lgr;

2

, &lgr;

4

. . . &lgr;

2

N

M

) communicated on optical fiber B

2

. Recall that &lgr;

1

refers to individual spectral components at intervals of

k

⁢

⁢

Δ

⁢

⁢

λ

2

N

⁢

⁢

M

from 1528-1564 nm.

The second stage consisting of two parallel fiber based Mach-Zehnder interferometer

155

2

,

155

3

, are configured in a tree-structure to receive the groupings of sliced spectral components

185

1

,

185

2

on its corresponding input optical fibers A. Likewise, each of the second stage Mach-Zehnder interferometers “slices” the groupings of spectral components

185

1

,

185

2

into further groupings by transmitting every other “sliced” spectral components onto its output optical fiber C

1

-C

4

as follows: grouping

190

1

of spectral components &lgr;

1

, &lgr;

5

. . . &lgr;

2

N

M−3

on optical fiber C

1

; grouping

190

2

of spectral components &lgr;

3

, &lgr;

7

. . . &lgr;

2

N

M−1

on optical fiber C

2

; grouping

190

3

of spectral components &lgr;

2

, &lgr;

6

. . . &lgr;

2

N

M−2

on optical fiber C

3

; and grouping

190

4

of spectral components &lgr;

4

, &lgr;

8

. . . &lgr;

2

N

M

on optical fiber C

4

. Of course, this is accomplished by setting the FSR of the second stage Mach-Zehnder interferometers to have twice the value as those of the previous stage (FSR=4R=0.16 nm).

Now, the third stage consisting of four parallel Mach-Zehnder interferometers

155

4

-

155

7

(FSR=8R=0.32 nm) are likewise configured in a tree architecture so as to receive the groupings of sliced spectral components

190

1

-

190

4

from preceding Mach-Zehnder interferometers

155

2

-

155

3

. Again, each of the preceding groupings

190

1

-

190

4

of sliced spectral components is filtered in the foregoing described manner, with the groupings of the sliced spectral components appearing on output ports D

1

-D

8

(

115

1

-

115

8

), respectively, such that eight (8) groupings

195

1

-

195

8

of one hundred twenty-eight (128) sliced spectral components appear as follow:

Output Optical Fiber

Spectral Components

115

1

(D

1

)

(195

1

) &lgr;

1

, &lgr;

9

, &lgr;

17

, . . .

115

2

(D

2

)

(195

2

) &lgr;

5

, &lgr;

13

, &lgr;

21

, . . .

115

3

(D

3

)

(195

3

) &lgr;

3

, &lgr;

11

, &lgr;

19

, . . .

115

4

(D

4

)

(195

4

) &lgr;

7

, &lgr;

15

, &lgr;

23

, . . .

115

5

(D

5

)

(195

5

) &lgr;

2

, &lgr;

10

, &lgr;

18

, . . .

115

6

(D

6

)

(195

6

) &lgr;

6

, &lgr;

14

, &lgr;

22

, . . .

115

7

(D

7

)

(195

7

) &lgr;

4

, &lgr;

12

, &lgr;

20

, . . .

115

8

(D

8

)

(195

8

) &lgr;

8

, &lgr;

16

, &lgr;

24

, . . .

For the sake of clarity, a depiction of the individual spectral components appearing at each stage as well as at each output optical fiber

115

1

-

115

8

is depicted in FIG.

8

.

Each of the 0.04 nm wide individual sliced spectral components in each of the eight bands

195

1

-

195

8

is then allocated a unique time slot using two sets of delay lines, implemented preferably using fibers. Note that the delay is equal to the propagation time in circulating through the corresponding length of fiber. A first set of delay lines

200

2

-

200

8

located between the 2

N

−1=7 optical fibers

115

2

-

115

8

and fiber Bragg gratings (

125

2

)

1

. . . (

125

1

)

128

through (

125

8

)

1

. . . (

125

8

)

128

delays each spectral component by a multiple integer of, for example, 0.8 nsec such that in the time domain the 1024 resolved spectral components are grouped into 128 groups

205

1

-

205

128

of eight nsecutively numbered sliced spectral components separated 0.8 nsec apart, as depicted in FIG.

9

. This is accomplished by employing an integer number of 16 cm length fibers for the spectral components, as follows:

Spectral

Output

Components

Delay Time

115

1

(D

1

)

(

205

1

)

⁢

⁢

∑

i

=

1

i

=

M

⁢

⁢

λ

i2

N

-

7

0 × 16 cm delay lines

(0 nsec)

115

2

(D

2

)

(

205

2

)

⁢

⁢

∑

i

=

1

i

=

M

⁢

⁢

λ

i2

N

-

6

4 × 16 cm delay lines

(3.2 nsec)

115

3

(D

3

)