Development of a Crystal-Type Optical Interleaving Filterby Mieko Yamagaki *, Sugio Wako *, Satoru Abe *,Daeyoul Yoon * and Hiroshi Matsuura *In recent years wavelength-division multiplexing (WDM) technology has been
adopted in configuring large-capacity long-haul fiber-optic communications networks, and bandwidths are becoming narrower in an effort to obtain higher densities on communication wavelengths. This has led to a need for interleaving filters, which from a single input candemultiplex alternate wavelengths to separate outputs. By developing a proprietary configuration,
the authors have succeeded in developing an optical interleaving filter that does not require temperature control. This paper describes the configuration of the module, and reports on the properties of prototypes that were fabricated.

ABSTRACT1.

INTRODUCTION

Since the beginning of the 1990s Internet use has spreadto all corners of the world, creating a massive increase in
the volume of communications traffic. In fiber-optic transmission systems, there have been major advances in
WDM technology, resulting in wide use of this method,which uses wavelength-division multiplexing to place multiple signals of different wavelengths on a single fiber.
Internet bandwidths have increased in recent years, butwith an on-going need for high-speed transmission oflarge volumes of data for still and moving images there is
every indication that there will be further leaps in communication traffic. To cope with this trend a technology is
being introduced which increases wavelength density anduses a larger number of channels. It is known as dense
WDM.
In DWDM systems narrow-band signals of from 50 GHz
to 25 GHz frequency spacing must be multiplexed withhigh accuracy and then demultiplexed into the component
signal wavelengths. This is done by devices known as
MUX/DEMUX modules using AWGs and filters, but withthese modules it is becoming extremely difficult technologically to increase densities above 50 GHz. Thus there is aneed for modules capable of taking signals multiplexed tosome extent by a MUX module and lumping them together
to achieve greater densities, and of demultiplexing dense
signals at broad frequency spacing.
Methods of realizing modules having these functions
include those using crystals and those using optical fibers,
but those using fibers suffer from a number of disadvantages: their characteristics are heavily temperature-dependent and since the fibers need to be accommodated to thedesigned routing, they are more bulky.
This paper reports on the development of a crystal-type
optical interleaving filter that can multiplex two signals at
100-GHz spacing to a spacing of 50 GHz, and demultiplex
the signal with 50-GHz spacing to a spacing of 100-GHz.

2.

BASIC CONFIGURATION

Figure 1 shows the basic configuration.
On one side of the light path determining section containing optical elements such as a birefringent crystal andFaraday rotator and the wavelength multiplexing/demultiplexing (MUX/DEMUX) section comprising an interference
filter, there are disposed three ports. In each port anaspherical lens is disposed at the end of a single-mode
fiber (SMF), so that the divergent light from the end of thefiber is collimated by the lens forming a connection
between the ports. The optical fiber and lens are fixed inplace by YAG welding and no adhesive is used in the lightpath for reasons of power resistance.
The light path determining section is of a non-reciprocal
structure, having an isolation function that prevents propa-Port-2
Port-1

Wavelength MUX/DEMUX section

Port-3

LensFerrule
SMF* Optical Components Development Dept., FITEL-Photonics
Lab.

Figure 1

Basic configuration of crystal-type optical interleaving filter.

1

gation of light in the reverse direction. By being selectivein the structure of the optical elements used in the lightpath determining section it is possible for them to be usedin either a multiplexing or demultiplexing module.
As an example, we will describe the propagation of lightduring demultiplexing.
When the wavelength multiplexed signal injected at Port
1 enters the wavelength MUX/DEMUX section it is separated by the interference filter to odd and even wavelengths that are both at the same frequency spacing. With
this the two parts of the divided signal are mutually crosspolarized, their optical paths are converted by the lightpath determining section, and they are output to Port 2
and Port 3 respectively.
In wavelength multiplexing, when signals from Port 2
and Port 3, each having the same frequency spacing anda wavelength shifted by one-half frequency spacing therespective signals are multiplexed and a further multiplexed signal with a frequency spacing one-half that of theinjected signal is output from Port 1.
Since the interference filter has ordinary temperature
characteristics, the peak wavelength of optical signals
multiplexed or demultiplexed by the crystal-type interleaving filter will vary with temperature. In the interleaving filter
described in this work, optical design is applied to compensate for the temperature dependence of the interference filter so that the module as a whole is of athermal
construction, with diminished temperature dependence ofthe peak center wavelength. Since temperature control is
not required, it is easier to handle and more cost-effective,
and the space that the temperature regulating circuitry
would have occupied is saved. This has made it possible
to design a module only 1/15 the volume of one with temperature regulation. The external appearance of the interleaving filter module is shown in Figure 2. Its dimensions
are 60 x 20 x 8.5 mm.

with wavelength multiplexing function. The wavelengths ofthe signals so injected at Ports 2 and 3 are each shiftedby one-half of the frequency spacing, or 50 GHz. Thesesignals are then lump multiplexed by the crystal-type interleaving filter into signals with a frequency spacing of 50
GHz. If the multiplexed signal is injected into Port 1 of acrystal-type interleaving filter having a wavelength demultiplexing function, a signal with 50-GHz spacing can be
demultiplexed into two signals with 100-GHz spacing.

4.

TARGET SPECIFICATIONSWith a view to achieving broad band and low insertion
loss, together with sufficient bandwidth and channel isolation for high-density transmission, the target specifications
shown in Table 1 were adopted.

5.

PERFORMANCE CHARACTERISTICS

5.1 Transmission Waveform
The transmission waveform of the crystal-type interleaving
filter is shown in Figure 4, and expansions of the waveform are shown in Figure 5(a) and 5(b) for the shorter andlonger wavelengths respectively. In the 1525-1565-nm
range, the transmission spectra of Ports 2 and 3 show acyclic spectrum with 100-GHz spacing. It can also be confirmed that the peak center wavelength between the twoports show an alternately shifted transmission characteristic.

λ1Multiplexing

λ3

λ3Port 23.

TYPICAL APPLICATIONS

Figure 3 shows an application of 100-GHz/50-GHz crystaltype optical interleaving filters in a DWDM system. Bymeans of an AWG or other multiplexing module signals
multiplexed at 100-GHz frequency spacing are injected
into Port 2 and Port 3 of the crystal-type interleaving filter

λN
λ2

Port 2

Port 1λN
λ2Port 1λ4

λ4Port 3

λ N+1Port 3

λ N+1Interleaving filter
MUXDEMUX

Figure 3Application of crystal-type optical interleaving filtersin DWDM system.

Table 1

Target specifications of optical interleaving filter.
ItemPort 2

λ1

Demultiplexing

Specification

Insertion loss@±10 GHz<1.5 dB

PDL@±10 GHz<0.4 dB

Transmission bandwidth@-0.5 dB

±10 GHz

Rejection bandwidth@-17 dB±5 GHzPort 3Isolation

Figure 22

External appearance of crystal-type optical interleaving filter.

>45 dB

Operating wavelength range λ op

1525~1565 nm

Operating temperature range Top0~65°C

Furukawa Review, No. 222002

00-2-4-6-10

Loss (dB)

Loss (dB)-5-15-8-10-12-14-20-16-18-251535

1540

1545

1550

1555

-201563

Wavelength (nm)1563.51564

1564.51565

Wavelength (nm)Figure 4

Transmission waveform of selected interleaving filter.

Figure 6

Temperature dependence of transmission waveform.

70-2

6

Port 2 to Port 1

5Port 3 to Port 1-4

Frequency

Loss (dB)-6-8-10-12-14

4
32-16
1-18

0-20
1525

1525.51526

1526.5

0.4

1527

0.6

0.8

1

1.2

1.4

Insertion loss (dB)

Wavelength (nm)a)1525-1527 nm

Figure 7

Histogram of insertion loss of MUX-type interleaving
filter.

0-2-4
5-8
Port 1 to Port 2

4-10

Port 1 to Port 3-12Frequency

Loss (dB)-6-14-16-18

32-20
1563

1563.51564

1564.51565

1Wavelength (nm)b)1563-1565 nm

00.4

Figure 5Figure 8Figure 6 shows the results of measurements of the temperature dependence of the transmission waveform of theinterleaving filter, together with a mask that shows transmission bandwidth and rejection bandwidth from Table 1.
The measurement temperatures ranged from 5 to 50°C,
and the range of temperature dependence of the peakcenter wavelength was ±1 pm/°C or less. From this graphwe can see that both the transmission and rejection bandwidths are substantially within the standard masks. The
range of temperatures was ±5°C narrower than the target,
but it was possible to confirm the reduction in temperaturedependence.
5.2 Insertion Loss Distribution
Figures 7 and 8 give histograms showing the distributionof insertion loss for multiplexer-type and demultiplexer-

0.6

0.8

1

1.2

1.4

Insertion loss (dB)Expansion of selected transmission waveforms.

Histogram of insertion loss of DEMUX-type interleaving filter.

type interleaving filters. An insertion loss of 1 dB or less
was achieved by more than 80% of the modules from Port
2 to Port 1 in the multiplexing type, and by more 100% of
the modules from Port 3 to Port 1 in the multiplexing type,as well as in the demultiplexing type.
5.3 Polarization-Dependent Loss (PDL)
Figure 9 gives a histogram showing the distribution ofpolarization dependent loss at room temperature. About
80% of modules shows a superior PDL characteristic of
0.04 dB or less.

3

5.4 Isolation
The relationship between isolation and wavelength isshown for a multiplexing interleaving filter in Figure 10
from Port 1 to Port 2, and in Figure 11 from Port 1 to Port
3. For all ports a high degree of isolation was achieved-45 dB or better at 1525-1565 nm, enough to satisfy operating requirements. The demultiplexing type also showedisolation of 45 dB or better in the operating wavelength
range.

5Multiplexing:
Port 2 to Port

4Frequency

Port 3 to Port 1
Demultiplexing:
Port 1 to port 23Port 1 to port 3210
0.02

0.04

0.06

0.08

0.1

PDL (dB)

Figure 9

Histogram of polarization dependent loss of interleaving filter.

0Isolation (dB)

-10-20-30-40-50-60
1535

1540

1545

1550

6.

CONCLUSION

A compact 50-GHz optical interleaving filter featuring low
insertion loss has been developed. It is of the athermal
type, which is recently in demand, and the temperaturedependence of peak center wavelength was held to 1pm/°C, and it thus promises to make a significant contribution to improving the performance of dense wavelengthdivision multiplexing (DWDM) systems. The next task is toconfirm reliability and to effect further improvements in thetemperature dependence characteristic of the peak center
wavelength. It is also proposed to develop optical interleaving filters using the same technology for the next generation of narrower-band (25 GHz) systems.

1555

Wavelength (nm)Figure 10 Isolation of MUX-type interleaving filter, Port 1 to Port
2.

0Isolation (dB)

-10-20-30-40-50-60
1535

1540

1545

1550

1555

Wavelength (nm)Figure 11 Isolation of MUX-type interleaving filter, Port 1 to Port
3.

4Furukawa Review, No. 222002