Regular Papers

Pass Band Broadening in Molded-glass
Echelon-grating-based CWDM Filter Module
Shigehisa Ohki†, Yuko Kawajiri, Makoto Fukuda,
Yoshiaki Tachikawa, and Mitsuhiro SatohAbstract
We have developed an inexpensive WDM (wavelength division multiplexing) filter module by using
a molded-glass echelon grating as the key component. This grating is designed to provide the wide transmission pass band (>13 nm) required by a CWDM (coarse WDM) filter. The body of the module and theferrule that secures the optical fibers are fabricated by injection molding. The volume of the module isapproximately 5.5 × 5.5 × 28.0 mm3, which is almost the same as that of an MU connector. The fabricated CWDM filter module is designed to handle four optical signals whose wavelengths are 1511, 1531,1551, and 1571 nm. The measured insertion loss and 1-dB-down bandwidth of the module are 5 dB and13.5 nm, respectively. The crosstalk performance is better than –17 dB. The channel center wavelengthis highly stable (within ±0.5 nm) over a wide range of ambient temperature from –40 to +85°C.

1. IntroductionWavelength division multiplexing (WDM) is a vitaltechnology in optical telecommunications systems.
The top of the network hierarchy utilizes narrowly
spaced, densely packed WDM called DWDM (denseWDM). However, the use of DWDM is still limited tothe top layer because of the very high cost of stabilizing the wavelength of the light that carries signals
through the transmission line. The solution to thisproblem is to use filters that have a wide, flat passband, which enables operation even when the wavelength of the optical signals varies within a certainrange of the band. This idea led to CWDM (coarseWDM), which has the potential to decrease systemcosts by eliminating the expensive wavelength stabilizing techniques.
The most popular optical filters that can meet therequirements for CWDM are dielectric thin film filters. We previously reported the basic characteristics† NTT Microsystem Integration Laboratories
Atsugi-shi, 243-0198 Japan
E-mail: ohkis@aecl.ntt.co.jp

50of a filter module employing an injection-molded
polymer echelon grating [1]. However, the transmission pass band was too narrow for CWDM because
we had not developed the design theory. Recently, wehave developed a new optical filter module that uses
a press-molded glass echelon grating. This module
offers the transmission characteristics required by
CWDM filters, such as a wide pass band, symmetric
demultiplexing and multiplexing operations, andhighly stable operation over a wide range of ambient
temperature.
2. Echelon grating filter
2.1 Grating
The principle of the echelon grating filter [2],which was first proposed by A. A. Michelson, can be
briefly summarized as follows. The structure consistsof a series of optically transparent planes stacked parallel to each other as shown schematically in Fig. 1.
The length of each plane monotonically decreases orincreases with a constant length difference, whichleads to interference among transmitted optical waves
in the focal plane. In deducing the function describNTT Technical Review

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ing the optical amplitude at the focal plane, let us consider the two adjacent optical paths I-I’ and II-II’ inFig. 1. The optical length difference D between thetwo is expressed as
D = l(n − cosθ ) + d sin θ ,(1)where l and d are respectively the length difference
parallel to and the height difference perpendicular tothe optical wave propagation. The angle θ is the diffraction angle, and n is the refractive index of thegrating material. Equation (1) can be rewritten to
express the amount of phase difference ∆φ as
2πd2πl(2)sin θ +(n − cosθ ).
λλAssuming an echelon grating consisting of N steps,
we can obtain the optical amplitude in the focal planeby summing the intensity of optical waves passing
through each tier. Therefore, the optical amplitude
should have the form [3]∆φ =Nu(r) = ∑ Em (r )exp[ − j∆φ m (r )],(3)m =1where j is an imaginary unit, r denotes the position
vector that terminates at the point in the focal plane,
and Em(r) and ∆φm(r) express the absolute value andthe phase originating from the partial light that passes through the m-th tier. To make a wide transmission
pass band, the parameters d and l must be modified tohave the specific values given to the m-th tier. Thus,

they should be expressed as dm and lm in Eq. (3).
2.2 Filter characteristics
The structure of our CWDM filter module is shown
schematically in Fig. 2. The module basically consists of inlet and outlet optical fibers, a pair of lenses
that form the collimated optical system, and the echelon grating. As the module is designed to handle
four optical signals having different CWDM-spaced
wavelengths, the four outlet fibers are gathered
together by a ferrule. In the demodulation (DEMUX)
operation, an optical beam comprising four channelsof signals/wavelengths is collimated by the first lensand illuminates the echelon grating. The grating
demultiplexes the signal into four separate opticalsignals and focuses them at different positions in thefocal plane. Each of the optical fibers receives one of
the four optical signals. The multiplexing (MUX)
operation is the same in reverse.
The transmission spectral responses of the module
can be calculated using Eq. (3). An example of thecalculated transmission spectral response of an echelon-grating-based optical filter module is shown inFig. 3. In the calculation, the center wavelengths ofthe four channels were set to 1511, 1531, 1551, and1571 nm. The calculated response of the grating,which is intended to provide the basic CWDM characteristics, exhibited 13.5-nm-wide flat transmission(1-dB-down bandwidth) and crosstalk performanceof better than –17 dB throughout the band.
3. Experimental results

I’lII’Iθ

IIdFig. 1. Schematic illustration of an echelon grating.

Figure 4 is a photograph of an assembled CWDM
filter module, which is of the inline type. It shows,from the left-hand side, a single inlet optical fiber, acollimating lens, an echelon grating, a condenser
lens, and a four-fiber outlet array. The enclosure or
the body of the module was fabricated by injection
molding and made of a plastic commonly used forferrules. Thanks to the small size of the grating andthe inline arrangement of the module structure, theλ1

λ1, λ2, λ3, λ4

λ2
λ3
λ4

Single fiber

Collimating lensEchelon grating

Condenser lensFiber array

Fig. 2. Basic structure of the echelon-grating-based optical filter module.

Vol. 3 No. 6 June 200551

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Power (dB)

0–10–20–30
1490

1510

1530

1550

1570

1590

Wavelength (nm)Fig. 3. Calculated transmission spectral response of the filter module.
Collimating lensEchelon grating

Condenser lensglass echelon grating contribute almost equally to theloss. We think that it should be possible to reduce theinsertion loss to well below 3.0 dB by improving theoptical connection scheme and the fabrication accuracy of the grating.
Another important characteristic of the optical filter
module is stability against variations in environmental conditions, especially temperature. The wavelength of the central channel is plotted against ambient temperature ranging from –40 to +85°C in Fig. 6.
The wavelength variation was less than ±0.5 nm. This
indicates that the module is highly stable over a wide
range of ambient temperature.
4. Discussion

Fig. 4. Photograph of a completed filter module.

volume of the module is only 5.5 × 5.5 × 28.0 mm3.
The volume of the grating is far less than 1 mm3.
The transmission spectral responses were measured
in the wavelength range from 1480 to 1600 nm using
a tunable laser source and an optical spectrum analyzer. Assigning the first channel as 1511 nm, we
obtained wavelengths of 1531, 1551, and 1571 nm forthe second, third, and fourth channels, respectively.
The transmission spectral response of the module
operating in DEMUX mode is shown in Fig. 5. The
measured 1-dB-down pass bandwidth and crosstalkwere 13.5 nm and –17 dB, respectively. TheseCWDM characteristics are almost the same as thedesigned values. At present, the insertion loss of ourmodule is approximately 5.0 dB. The excess loss
appearing in the optical connection and the molded52

Initially, we made an injection-molded polymer
echelon grating because injection molding can easily
fabricate sophisticated structures such as echelon
gratings. However, we should also consider that polymer materials generally have a large temperaturedependence in material expansion and/or refractive
index. This dependence leads to a large change in thecentral wavelength of a channel when the ambient
temperature changes, which we confirmed experimentally. This weakness makes our first design ofechelon grating-based filters unsuitable for CWDM.
The most promising solution was to use glass to form
the gratings because glass has comparatively low
thermal dependences. Thanks to advances in the highprecision molding, we were able to overcome the difficulties of forming glass echelon gratings and thefabrication cost was almost the same as that for polyNTT Technical Review

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0
Ch 1
Ch 2
Ch 3
Ch 4–5

Power (dB)

–10–15–20–25–30
1480

1500

1520

1540

1560

1580

1600

Wavelength (nm)Fig. 5. Example of the measured transmission spectral response.
1580

Channel center wavelength (nm)

1570
1560
1550
1540
1530
1520
1510
1500
–50

050100

Ambient temperature (°C)

Fig. 6. Relationship between the channel center wavelength and ambient temperature. The variation of the channel
center wavelength was less than ±0.5 nm.

mer injection molding.
However, low thermal expansion and optical index
change coefficients are not enough to produce highly
stable operation over a wide range of ambient temperature. We think our excellent operating characteristics were also aided by other factors: (1) the coaxial arrangement of components along the optical axis,(2) the symmetrical design of the body around theoptical axis, and (3) the use of the same materials forVol. 3 No. 6 June 2005

the body and the ferrule (plastic) and for the echelon
grating and lenses (glass). Although the contributionof each factor to stable operation is not clear at present and should be clarified quantitatively in the near
future, these temperature-tolerant transmission characteristics make our molded-glass echelon-gratingbased filter highly competitive among commerciallyavailable filters.

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5. Conclusion
We reported the fabrication and characteristics of anew compact four-channel CWDM filter module
employing a molded-glass echelon grating. Press
molding and injection molding techniques were used
to reduce manufacturing costs. The filter module
multiplexes and demultiplexes multiple optical signals using only one echelon grating. The measured
pass bandwidth and crosstalk performance were 13.5
nm and –17 dB, which coincide very well with thedesigned values. The obtained insertion loss wasaround –5.0 dB, which is somewhat large. However,it will be reduced by improving the optical connection and the fabrication accuracy of the echelon grating. The best feature of the module is its highly stable

operation over a wide range of ambient temperature.
The characteristic performances of the grating in thismodule show that the echelon grating is promisingfor developing low-cost high-performance CWDM
filter modules.
References
[1]

[2]
[3]

S. Ohki, Y. Tachikawa, Y. Kawajiri, and H. Takahara, “A New
CWDM Filter Module Employing a Polymer-Molded Echelon Grating,” Proceedings of 11th International Workshop on Optical Hybrid
Access Networks, Full Service Access Networks, Full Service VDSL
Initiative, pp. 109-114, 2002.
M. Worn and E. Wolf, Principles of Optics (Pergamon Press, Oxford,1970) 4th edition, pp. 409-411.
Y. Tachikawa, “Spectral Analysis of Transmission Echelon Grating
Filters for Photonic Networks,” OPTICAL REVIEW, Vol. 6, No. 2,
pp. 131-138, 1999.

Shigehisa Ohki

Yoshiaki Tachikawa

Group Leader, Senior Research Engineer,Supervisor, Photonic Interconnection ResearchGroup, Network Hardware Integration Laboratory, NTT Microsystem Integration Laboratories.
He received the M.E. and D.E. degrees in electronics from Nagaoka University of Technology,Nagaoka, Niigata in 1982 and 1994, respectively. In 1982, he joined the Electrical Communications Laboratories of Nippon Telegraph andTelephone Public Corporation (now NTT). He
worked on the development of LSI fabrication
process and X-ray lithography technologies.
Since 2001, he has been engaged in R&D of photonic interconnection technology. He is a member of the Japan Society of Applied Physics(JSAP) and the Physical Society of Japan.

Senior Research Engineer, Network Hardware
Integration Laboratory, NTT Microsystem Integration Laboratories.
He received the B.S. and M.S. degrees in electrical engineering from Tokyo Denki University,
Tokyo in 1975 and 1978, respectively. In 1978,
he joined the Electrical Communications Laboratories of Nippon Telegraph and Telephone Public Corporation (now NTT), where he worked onelectro-optical converter design and laser diodemodule design. From 1984 to 1995, he wasengaged in the development of bulk-type WDM
transceiver modules for optical subscriber systems, and silica-based arrayed-waveguide grating add-drop multiplexers and their applications.
From 1996, he engaged in research on fiber gratings and polymer waveguide gratings. Currently,
he is engaged in the development of opticaldevices for WDM systems. He is a member ofIEICE and JSAP.

Yuko Kawajiri
Research Engineer, Network Hardware Integration Laboratory, NTT Microsystem Integration Laboratories.
She received the B.S. and M.S. degrees inchemistry from Gakushuin University, Tokyo in
1993 and 1995, respectively. In 1995, she joined
NTT Interdisciplinary Research Laboratories,
where she engaged in research on passive opticalinterconnection devices. Currently, she isengaged in R&D of optical devices for WDM
systems. She is a member of the Institute of Electronics, Information and Communication Engineers (IEICE) of Japan.

Mitsuhiro SatohChief, NTT Advanced Technology Corporation.
He graduated from Naruto technical seniorhigh school, Naruto, Tokushima in 1987. He
joined NTT Advanced Technology Corporationin 2002. Since 2002, he has been engaged in thedevelopment of optical devices for WDM systems.

Makoto Fukuda
Senior Research Engineer, Supervisor, Network Hardware Integration Laboratory, NTT
Microsystem Integration Laboratories.
He received the B.E. degree in control engineering and the M.E. and D.E. degrees in precision mechanical engineering from Tokyo Instituteof Technology, Tokyo in 1978, 1981, and 1998,
respectively. In 1981, he joined the ElectricalCommunications Laboratories of Nippon Telegraph and Telephone Public Corporation (nowNTT). He has been engaged in R&D of LSI manufacturing and precision mechanical control
including an X-ray exposure system. He is currently a professor in the Department of Intelligent
Machines and System Engineering in Hirosaki
University. He received the Best Poster Award of
Micro- and Nano-Engineering in Europe, 1993.
He is a member of the Japan Society for Precision
Engineering (JSPE), the Japan Society of
Mechanical Engineers (JSME), JSAP, and IEICE.

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