Development of a Low-Loss Optical Circulator
by Yoji Makiuchi * and Hiroshi Matsuura *2

The optical circulator is one of the key devices in the optical add-drop modules(OADMs) used in wavelength-division multiplexing (WDM) technology, whichfinds applications in large-capacity long-haul telecommunications systems. A low-loss optical circulator has been developed to significantly improve OADM performance. Insertion loss for thesemodules is extremely low--less than 0.44 dB at the operating temperature and wavelength range.
High reliability and high power resistance are assured by fabrication techniques that use YAG
welding for mechanical components, eliminating the use of adhesives in the optical path.

ABSTRACT1.

INTRODUCTION

The Internet, which in the 1990s spread primarily in North
America, has undergone explosive growth in recent years.
What were originally only local networks have expanded
to a worldwide scale, and the data sent and received hasgone from text only to still and moving images, requiring
quantum jumps in capacity. In wide-area networks fiberoptics has become indispensable in the instantaneoustransmission of large volumes of data.
Fiber-optic networks, which heretofore have been confined to universities, research institutes and a few large
corporations have, with the spread of the Internet,advanced rapidly into ordinary homes. This has created aneed for the more effective utilization of fiber-optic networks, and one way of accomplishing this has been withwavelength division multiplexing (WDM) technology, inwhich a number of signals of differing wavelength can becarried on a single fiber.
The optical circulator is one of the key devices used byWDM-type OADMs, which extracts and multiplexes thedesired wavelengths. As networks have expanded inrecent years and the distances that signals are transmitted have lengthened, the need to decrease the loss ineach individual module as a means of sending the signals
further has become more necessary than ever before.
And since these modules can be used in bi-directional
amplifiers and a wide range of other equipment, in addition to OADMs, reduction of loss has come to be an
essential attribute.
This paper presents the fundamental principles of the
optical circulator, and goes on to report on development ofa marketable 3-port optical circulator that achieves low
loss by optimizing losses between the various ports. It**2

Optical Components Dept., FITEL Products Div.
Optical Components Development Dept., FITEL-Photonics
Lab.

also reports on the development of a 4-port optical circulator that put into application the principles of the 3-porttype.

2.

CONFIGURATION

Figure 1 is a schematic diagram of the newly developed
low-loss 3-port optical circulator. It comprises three singlemode fibers (SMFs), single-fiber ferrules, lenses and a
non-reciprocal section using a uniaxial birefringent crystal
and garnet. The various components are held in place bylaser welding and adhesive bonding, but to achieve highpower resistance no adhesive is used in the optical path.

Single-fiber ferrule

Lens
Single-fiber ferrule

Port 1

Port 2
Lens

Port 3

Non-reciprocal section

Figure 1

Schematic diagram of newly developed optical circulator.

Figure 2

Appearance of newly developed optical circulator.

1

Circulator 1

8Circulator 2
Port 2

Port 1

Port 2-5°CPort 3

FBG25°C

Port 3Figure 3

Frequency

6Port 1

Schematic diagram of use of optical circulators in anOADM.

70°C

4200.25

Table 10.3

Specifications of newly developed optical circulator.
Item

I/O ports
1 to 2Insertion loss
2 to 32 to 1Isolation
3 to 2

0.35

0.4

0.45

0.5

0.55

Insertion Loss (dB)

RatingFigure 4

Histogram of insertion loss from port 1 to port 2.

≤0.5 dB
6-5°C≥30 dB5≥55 dB

425°C

1 to 3
1 to 2PDL
2 to 3Return loss

1, 2 & 3

Frequency70°C

Directivity≤0.1 dB

32≥55 dB
1Rated wavelength range

1525~1570 nm

Dimensions (L × W × T)

71.5 × 11 × 6 mm

00

0.02

0.04

0.06

0.08

0.1

0.12

PDL (dB)

Following is a description of module function based on
Figure 1. The light injected through port 1 is collimated at
the lens, passes through the non-reciprocal section, and isfocused at the lens of port 2 and output to port 2. Similarly
the light injected through port 2 is collimated at the lens,passes through the non-reciprocal section, and is focusedat the lens of port 3 and output to port 3. Figure 2 shows
the appearance of the optical circulator.
2.1 Example Application
Figure 3 is a schematic diagram showing the use of optical circulators in an OADM.
When the multiplexed signal is input to port 1 of circulator 1 it flows through port 2 to a fiber Bragg grating (FBG)
that reflect only certain wavelengths. The signal lightreflected from the FBG is returned to port 2 of circulator 1and output (or dropped) from port 3. The other signals
pass though, being input to port 2 of circulator 2 and output to port 3. In the case of multiplexing, the added signal
is input from port 1 of circulator 2. The signal output fromport 1 to port 2 is reflected by the FBG, passing again
through port 2 and being output to port 3. At this time it ismultiplexed (added) to the signal passing through theFBG.

3.

SPECIFICATIONS

In keeping with past designs aimed at lower loss, the targets set forth in Table 1 were set for all operating temperatures (-5 to 70°C) and a full range of wavelengths.

2

Figure 54.

Histogram of PDL from port 1 to port 2.

OPTICAL CHARACTERISTICS

4.1

Insertion Loss, Temperature Characteristics andPDLThe port numbers correspond to those shown in Figure 2.
Insertion loss and PDL are extremely important for thismodule, and indeed for any component used in fiber-optic
communications. Figures 4 and 5 show histograms ofinsertion loss and PDL from port 1 to port 2, and Figures 6and 7 give analogous histograms for port 2 to port 3.
Insertion losses were very low, averaging only 0.36 dB
from port 1 to port 2, and 0.39 dB from port 2 to port 3.
This was slightly higher than the value from port 1 to port
2 because of the use of an optical path converting element.
For PDL, the values averaged 0.04 dB for both port 1 to2 and port 2 to 3, fully satisfying the specifications.
Figures 8 and 9 show typical wavelength dependence ofinsertion loss at selected temperatures for port 1 to 2 andport 2 to 3. The change in the characteristics due to temperature was very low--less than 0.05 dB. The wavelengthchange was also low--less than 0.1 dB from port 1 to port
2 and less than 0.08 dB from port 2 to 3.
4.2 Isolation
Figures 10 and 11 show histograms of isolation from port
2 to port 1 and from port 3 to port 2. When isolation issmall the returned light influences the original signal, andFurukawa Review, No. 22

2002

56-5°C

5-5°C

25°C

25°C

70°C

3Frequency

Frequency

421470°C

3
2
1

00
0.25

0.3

0.35

0.4

0.45

0.5

40

0.55

4448

Insertion Loss (dB)Figure 65660

Isolation (dB)Histogram of insertion loss from port 2 to port 3.

Figure 10 Histograms of isolation from port 2 to port 1.

66-5°C-5°C
5525°C25°C

70°C

70°C

Frequency

4Frequency

52

34322110000.02

0.04

0.06

0.08

0.1

40

0.12

44

48

56

60

Isolation (dB)

PDL (dB)

Figure 752

Histogram of PDL from port 2 to port 3.

Figure 11 Histograms of isolation from port 3 to port 2.

700.50.3

0.2
Port 1 to Port 2 at 25°C
Port 1 to Port 2 at 70°C
Port 1 to Port 2 at -5°C

0.1050403020Port 1 to 3 at 25°C
Port 1 to 3 at 70°C
Port 1 to 3 at -5°C

1001525

1540

1555

1570

1525

1540

1555

1570

Wavelength (nm)Wavelength (nm)Figure 8Directivity (dB)

Insertion Loss (dB)60
0.4

Wavelength dependence of insertion loss at selected
temperatures from port 1 to port 2.

Figure 12 Wavelength dependence of directivity at selected
temperatures from port 1 to port 3.

Insertion Loss (dB)

0.5

0.4

from the principle of the optical circulator is temperaturedependent, but by optimizing the design of this module it
was possible to fully satisfy the specifications with anaverage of 47 dB.

0.3

0.2
Port 2 to Port 3 at 25°C
Port 2 to Port 3 at 70°C
Port 2 to Port 3 at -5°C

0.101525

1540

1555

1570

Wavelength (nm)Figure 9

4.3 Directivity
Figure 12 shows the wavelength dependence of directivity
at selected temperatures from port 1 to port 3. Thechange due to both temperature and wavelength was 55
dB or more, satisfying the specifications.

Wavelength dependence of insertion loss at selectedtemperatures from port 2 to port 3.

34.4 Return Loss
Figure 13 shows a histogram of return losses at each of
the ports. The average for all ports was 63 dB, fully satisfying the specifications.

9Port 1

8Port 2

7Frequency

Port 365435.

21

In the present work a 4-port circulator was also developed
by applying the technology used in the 3-port design.
Figures 14 through 16 show the wavelength dependence
of insertion loss at selected temperatures for a 4-port circulator. It was possible to realize a module having aninsertion loss of less than 0.53 dB from port 1 to 2 andport 2 to 3, and less than 0.6 dB from port 3 to 4. The
specifications shown in Table 1 were also fully satisfied forcharacteristics other than insertion loss.

054

58

6266

70

74

Return Loss (dB)Figure 13 Histogram of return losses.

0.6

Insertion Loss (dB)

0.5
0.4
0.3

6.

CONCLUSION

0.2
Port 1 to Port 2 at 25°C
Port 1 to Port 2 at 70°C

0.1

Port 1 to Port 2 at -5°C
01525

1540

1555

1570

Wavelength (nm)Figure 14 Wavelength dependence of insertion loss at selected
temperatures for 4-port circulator from port 1 to port 2.

0.6
0.5

Insertion Loss (dB)

4-PORT CIRCULATOR

0.4

Based on the fundamental principle of the optical circulator, it was possible, by optimizing losses between the various ports, to develop a low-loss 3-port circulator with an
insertion loss of less than 0.44 dB within the range of theoperating temperatures and wavelengths. The realizationof these low-loss characteristics promises to improve theperformance both of OADMs and of a wide range of otherequipment such as bi-directional amplifiers.
It was also possible, applying the technology used in the3-port circulator, to develop a 4-port circulator with aninsertion loss of less than 0.6 dB. This will make it possible to meet rising expectations with respect to 4-port circulators.

0.3

ACKNOWLEDGMENT

0.2
Port 2 to Port 3 at 25°C
Port 2 to Port 3 at 70°C

0.1

Port 2 to Port 3 at -5°C
01525

1540

1555

1570

Wavelength (nm)The authors would like to express their appreciation for
the invaluable assistance and cooperation rendered inconnection with the preparation of this report by theOptical Functional Components Group, and the OpticalComponents Dept. for the FITEL Products Div.

Figure 15 Wavelength dependence of insertion loss at selected
temperatures for 4-port circulator from port 2 to port 3.

0.6Insertion Loss (dB)

0.5
0.4
0.3
0.2
Port 3 to Port 4 at 25°C
Port 3 to Port 4 at 70°C
Port 3 to Port 4 at -5°C

0.1
01525

1540

1555

1570

Wavelength (nm)Figure 16 Wavelength dependence of insertion loss at selected
temperatures for 4-port circulator from port 3 to port 4.

4Furukawa Review, No. 222002