We propose and demonstrate a widely tunable optical filter, realized by angle tuning a volume holographic grating. The volume holographic grating selectively drops a narrow portion of the signal bandwidth into a fiber while passing through the rest of the signals. The demonstrated 1510- to 1590-nm tuning range covers the entire erbium-doped fiber amplifier (EDFA) C band, with small bandwidth variation and low insertion loss (<1 dB). Group delay, polarization-dependent loss, and polarization mode dispersion are measured and investigated for optimizing the filter characteristics

Havermeyer, Frank

Liu, Wenhai

Moser, Christophe

Psaltis, Demetri

Steckman, Gregory J.

Caltech Authors - Main

DownlVolume holographic grating-based continuously
tunable optical filter
Frank Havermeyer
Wenhai Liu
Christophe Moser
Ondax, Incorporated
850 East Duarte Road
Monrovia, California 91016
E-mail: havermeyer@ondax.com
Demetri Psaltis, FELLOW SPIE
California Institute of Technology
Department of Electrical Engineering
Pasadena, California 91125
Gregory J. Steckman
Ondax, Incorporated
850 East Duarte Road
Monrovia, California 91016
Abstract. We propose and demonstrate a widely tunable optical filter,
realized by angle tuning a volume holographic grating. The volume ho-
lographic grating selectively drops a narrow portion of the signal band-
width into a fiber while passing through the rest of the signals. The dem-
onstrated 1510- to 1590-nm tuning range covers the entire erbium-
doped fiber amplifier (EDFA) C band, with small bandwidth variation and
low insertion loss (,1 dB). Group delay, polarization-dependent loss,
and polarization mode dispersion are measured and investigated for op-
timizing the filter characteristics. © 2004 Society of Photo-Optical Instrumentation
Engineers. [DOI: 10.1117/1.1773775]
Subject terms: tunable optical filter; volume holographic grating; wavelength-
division multiplexing.
Paper VHOE-S06 received Nov. 30, 2003; revised manuscript received Mar. 23,
2004; accepted for publication Apr. 9, 2004.1 Introduction
Wavelength-division multiplexing ~WDM! has extended fi-
ber optics from simple point-to-point links to flexible large-
capacity networks capable of dynamic wavelength routing
for efficient capacity management. Such dynamic optical
networks require reconfigurable optical components to con-
trol the optical channels for optimum network per-
formance.1 One key component in WDM networks is the
tunable optical filter, which provides the flexibility in wave-
length control and routing. Its essential requirements in-
clude wide tuning range, low insertion loss, low chromatic
dispersion, low polarization-dependent loss ~PDL!, low po-
larization mode dispersion ~PMD!, high stopband rejection,
fast tuning speed, small size, and low cost.2
Available single-passband tunable optical filter tech-
niques include fiber Bragg gratings ~FBGs!,3 thin film fil-
ters ~TFTs!,4 Fabry-Perot filters ~FPF!,5 arrayed waveguide
gratings ~AWGs!,6 acousto-optic tunable filters ~AOTFs!,7
and microelectro-mechanically-actuated tunable filters.8
The FBG gives the best tunable optical filter because of its
sharp spectral filtering characteristics, low insertion loss,
and chromatic dispersion compensation capability with
proper chirping and apodization along the grating. How-
ever, the FBG is difficult to be tuned for wide range by
either thermal tuning or mechanical stress.
Volume holographic gratings ~VHGs! are in many ways
similar to FBGs except that the recording medium for the
grating is not a single-mode fiber but a volume medium.
Consequently, the incident and diffracted light are not con-
fined to the modes of the fiber, but can be assigned to any
mode that can propagate in the volume of the material. This
opens up a new set of possibilities for the design of useful
devices, and allows additional flexibility such as removal of
the circulator, which is essential for FBGs. VHGs have
been previously used as optical filters9 and de-
multiplexers.10,11 We investigate and demonstrate a VHG-Opt. Eng. 43(9) 2017–2021 (September 2004) 0091-3286/2004/$15.00
oaded From: http://astronomicaltelescopes.spiedigitallibrary.org/ on 09/30/20based continuously tunable filter. It achieves a wide tuning
range ~1510 to 1590 nm! and low insertion loss ~,1 dB!.
2 Volume Holographic Grating Angle Tuning
Figure 1 shows a schematic representation of the continu-
ously angle-tunable volume holographic filter. The Bragg
wavelength of a holographic grating is determined by:12
lb52nL cos u , ~1!
where lb is the Bragg phase matching wavelength, n is the
refractive index of the material, L is the holographic grat-
ing period, and u is the angle of incidence inside the holo-
graphic material. By changing the incident angle u, the
Bragg wavelength can be tuned continuously from 2nL
down to 2LA(n221). For practical application, the tuning
range can easily cover the entire C band, limited mainly by
the higher insertion loss at larger angle u, which is caused
by: 1. the sharp angle selectivity of the VHG at a large
angle u compared with the angle deviation of a finite aper-
ture Gaussian beam, and 2. the coupling efficiency between
a large aperture Gaussian beam and fiber due to the geom-
etry limits. The critical requirement to make this idea a
practical tunable filter device is to collect the drop signal
into a fiber during the angle tuning without an expensive
tracking mechanism or feedback control system. This is
achieved with a self-reflector architecture that recombines
the reflective holographic grating with a wideband infrared
~IR! mirror, as shown in Fig. 1.
A retroreflector consists of two mirrors with a fixed
angle a between the mirror surfaces. And the angle be-
tween the input and reflected signal becomes p22a. For a
normal retroreflector, as shown in Fig. 1~a!, the IR mirrors
are arranged as orthogonal to each other with a5p/2, and
the reflected signal beam is always inverse to the input
direction and fixed spatially when rotating the mirror struc-2017© 2004 Society of Photo-Optical Instrumentation Engineers
16 Terms of Use: http://spiedigitallibrary.org/ss/termsofuse.aspx
Havermeyer et al.: Volume holographic grating-based . . .
Downlture around the crossing point of the mirror surfaces. This
also applies to the structure in Fig. 1~b!, where one mirror
is replaced by a VHG. At the Bragg wavelength, this VHG
works as a reflective mirror with an effective depth into the
material. When rotating the mirror/grating structure around
the crossing point of the mirror and grating surfaces at the
effective reflection depth, the drop signal beam is fixed spa-
tially while the wavelength is tuned by the angle u.
With the addition of a temperature sensor and suitable
control system, the device can be made athermal13 by com-
pensating the angle u to offset temperature-induced varia-
tions of the grating period L in response to ambient tem-
perature variation.
3 Insertion Loss, Beam Size, and Experimental
Demonstration
Figure 2 shows an experimental demonstration of continu-
ous tuning of such a device across the C band. The tuning
range is only determined by the geometry limit of the de-
vice and the filter performance. The tunable device consists
of two parallel fiber collimators with beam width 500 mm
as input and drop, an IR mirror, and a holographic grating
of fixed angle a;90 deg, as shown in Fig. 1. A tunable
laser is coupled into the input fiber collimator and the dif-
fracted signal is collected into the drop fiber collimator. The
holographic grating is recorded inside a photosensitive
glass with period 535 nm, which corresponds to a Bragg
wavelength 1591.5 nm at normal incident angle. The angle
u is tuned between 11 and 17 deg to cover the C band 1560
to 1528 nm, as shown in Fig. 2~a!.
Figure 2~b! shows all the overlapped filter shapes over
the whole tuning range and they are similar with limited
deformation. However, the insertion loss ~IL! increases
from 2.2 to 4 dB while tuning across the C band in Fig.
2~a!. This increase of IL is due to the finite aperture of the
input signal. For an input Gaussian beam with a finite beam
size w, there is a spatial angle deviation ;l/w from a
perfect plane wave. However, the angle selectivity of a re-
flective grating Du5l/(2n sin uL) improves as the tuning
incident angle u increases, where L is the effective length of
Fig. 1 (a) A normal retroreflector with two perpendicular mirrors.
The reflected beam is fixed when the retroreflector is rotated around
the mirror joint. (b) Replacing one mirror by a VHG. The continu-
ously tunable wavelength of this optical filter is determined by the
VHG incident angle u.2018 Optical Engineering, Vol. 43 No. 9, September 2004
oaded From: http://astronomicaltelescopes.spiedigitallibrary.org/ on 09/30/20the grating. When Du is comparable with or smaller than
the Gaussian angular deviation, part of the Gaussian beam
will not be Bragg matched at the same time as the central
components.
By simulating the Bragg diffraction of different Gauss-
ian beam sizes from the holographic grating, Fig. 3 shows
that the IL of a 500-mm-diam input beam increases 1.5 dB
along the tuning angle 10 to 18 deg. To decrease the IL
variation ,0.5 dB over the C band, the collimated beam
size needs to be .1.5 mm in diam. Figure 4 shows the
experimental tuning filters with similar devices in Fig. 1
with collimators of 2.5-mm beam size. The tuning range
covers 1510 to 1590 nm, and the IL variation is smaller
than 0.7 dB, as shown in Fig. 4~b!. The larger than expected
IL variation is random instead of systematically going
down as the tuning angle increases, which could be contrib-
uted from other measurement noise such as the stability of
the angular tuning and the extreme angular coupling sensi-
tivity of the large beam collimator. The nonsymmetric filter
shape, as shown in Fig. 4~b!, is possibly caused by the
nonuniform refractive index along the grating due to the
Fig. 2 (a) Continuous tuning filters over C-band with fiber collima-
tors of 0.5-mm-diam beam size; (b) overlap of all the filter shapes
with limited variation over the whole tuning range.16 Terms of Use: http://spiedigitallibrary.org/ss/termsofuse.aspx
Havermeyer et al.: Volume holographic grating-based . . .
Downlnonuniform apodized exposure in the holographic material
in our experiments, similar to the induced refractive index
variation in FBG. Figure 5 shows the filter bandwidth mea-
sured at both 20.5 and 23 dB from the maximum as the
Fig. 3 Theoretical simulation on the insertion loss over the tuning
angle for various Gaussian beam diameters.
Fig. 4 (a) Continuous tuning filters over 1510 to 1590 nm with fiber
collimators of 2.5-mm-diam beam size; (b) overlap of all the filter
shapes with IL ,1 dB and limited variation over the whole tuning
range.oaded From: http://astronomicaltelescopes.spiedigitallibrary.org/ on 09/30/20angle tuning across the C band. Less than 10% variation in
bandwidth is seen over the entire tuning range.
4 Group Delay, PDL, PMD Measurements, and
Discussion
Figure 6 measures the group delay of the tunable filter at
the center of the C band. The group delay ripple is smaller
than 5 ps within the 23-dB bandwidth. The bandwidth,
isolation, and other performance characteristics of the filter
can be tailored during fabrication to suit the application by
changing the grating strength, apodization, and chirping,14
in a manner similar to the FBG.
However, unlike the FBG, the VHG is a free-space de-
vice and the input beam at a nonzero incident angle sees
different effective grating strength for polarization in or out
of the incident plane of the grating. This could cause high
PDL and PMD in the volume holographic filter. We mea-
sured the PDL and PMD of the tunable filter at three dif-
ferent frequencies across the tuning range, as shown in Fig.
7. Within the tuning angle of 11 to 17 deg, PDL is smaller
than 0.4 dB and PMD is smaller than 0.4 ps. Both PMD
Fig. 5 Filter bandwidth while tuning over the C-band.
Fig. 6 Group delay measurement of the tunable filter at the center
of C-band.2019Optical Engineering, Vol. 43 No. 9, September 2004
16 Terms of Use: http://spiedigitallibrary.org/ss/termsofuse.aspx
Havermeyer et al.: Volume holographic grating-based . . .
Downland PDL increase as the filter is tuned to higher frequencies
due to the increase in the incident beam angle.
There are various methods used in optical networks to
improve the PDL and PMD.15 One potential solution to
minimize both PDL and PMD further is utilizing a multi-
pass architecture, in which the beam is diffracted by the
same or a different grating a second time after undergoing
90 deg of polarization rotation. Multiple passes through a
single grating can also be used to generate filters with small
frequency spacing and high stopband isolation. One such
approach is to replace the drop collimator in Fig. 1 with a
Faraday rotation mirror ~FRM!. The device is constructed
as previously described, but the first diffracted drop beam is
instead reflected by a FRM, which rotates the polarization
of the light as well as reflects it back along the original
path. The reflected beam then makes a second pass through
the VHG and is diffracted backward along the same path as
the original input light. Since the second pass is made with
light whose polarization is rotated with respect to that of
the first pass, polarization-dependent effects such as PMD
and PDL are reduced. However, a circulator is needed to
separate the output from the input, as the normal FBG.
There are also alternative double-pass configurations
achieved without using a circulator. One architecture is to
use a retroreflector to reflect the drop beam backward to the
grating, along a path transversely shifted from the drop
beam. This beam will then diffract a second time by the
VHG, reflect off the IR mirror, and be coupled into a sec-
ond collimator that is parallel to the first input collimator.
To compensate for PMD and PDL effects, a half-waveplate
can be placed in the beam path to exchange the in/out of
plane polarization components before making a second
pass through the VHG.
5 Conclusion
Volume holographic grating maintains the sharp filter per-
formance of a FBG while providing the addition freedom
of manipulating the signal in free space. With proper design
of an optical architecture, we demonstrate a simple single-
parameter tunable optical filter over 1510 to 1590 nm and
low insertion loss ,1 dB. By combining the mature
Fig. 7 Polarization dependent loss and polarization mode disper-
sion.2020 Optical Engineering, Vol. 43 No. 9, September 2004
oaded From: http://astronomicaltelescopes.spiedigitallibrary.org/ on 09/30/20apodization/chirping FBG technique and the flexible archi-
tecture, volume holographic gratings can lead to the con-
struction of more sophisticated continuous tunable filters,
such as low PDL, PMD, and dispersion filters, or dispersion
compensation.
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Frank Havermeyer received his diploma
and PhD degree in physics from the Uni-
versity of Osnabrueck (Germany) in 1996
and 2000, respectively. After two postdoc-
toral positions in Germany at the GKSS
Research Center and the University of
Bonn he joined the start-up company
Ondax (USA) in 2001. He is currently work-
ing on the design and optimization of the
production process of volume phase grat-
ings for DWDM and general wavelength fil-
ter applications.
Wenhai Liu received his BS from the Tsin-
ghua University (China) in 1992, MS in
physics from the University of Chicago in
1996, and PhD in electrical engineering
from the California Institute of Technology
in 2001. He has been working on product
research and development at Ondax, Inc.
since 2001. His research interests are ho-
lographic technology and their applications
in telecommunication and optical informa-
tion processing.16 Terms of Use: http://spiedigitallibrary.org/ss/termsofuse.aspx
Havermeyer et al.: Volume holographic grating-based . . .
DownlChristophe Moser is co-founder and CEO
of Ondax. Prior to founding Ondax, Dr.
Moser held management positions in the
optical measurement system division of
TESA-Brown & Sharpe in Switzerland, and
at Holoplex, an optical data storage
start-up in Pasadena, California. During his
thesis at Caltech, he developed devices
and processes that are at the core of
Ondax’s technology. He received his PhD
in electrical engineering from Caltech.
Demetri Psaltis is the Thomas G. Myers
Professor of Electrical Engineering at
Caltech in Pasadena, California. He was
educated at Carnegie-Mellon University
where he received a BS degree in electri-
cal engineering and economics in 1974, a
master’s in 1975, and a PhD in electrical
engineering in 1977. In 1980, he joined the
faculty at the California Institute of Technol-
ogy, Pasadena, California, and he served
as executive officer for the Computation
and Neural Systems Department from 1992 to 1996. From 1996oaded From: http://astronomicaltelescopes.spiedigitallibrary.org/ on 09/30/20until 1999 he was the director of the National Science Foundation
research center on Neuromorphic Systems Engineering at Caltech.
He is currently director of the Center for Optofluidic Integration at
Caltech. His research interests are in optical information processing,
holography, optical networks, imaging, optical memories, and optical
devices. He has authored or co-authored more than 300 publica-
tions in these areas. Dr. Psaltis is a fellow of the Optical Society of
America and SPIE. He received the International Commission of
Optics Prize and the Humboldt Award. He is a chairman and co-
founder of Ondax.
Gregory J. Steckman received his BS
from the University of California, San Di-
ego, in 1995 and his MS and PhD from the
California Institute of Technology in 1997
and 2000, respectively, all in electrical en-
gineering. He is currently the director of en-
gineering at Ondax, Inc., responsible for
the research and design of new products.
Dr. Steckman has authored or co-authored
numerous publications on the subject of
volume holography.2021Optical Engineering, Vol. 43 No. 9, September 2004
16 Terms of Use: http://spiedigitallibrary.org/ss/termsofuse.aspx


Volume holographic grating-based continuously tunable optical filter

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Volume holographic grating-based continuously tunable optical filter

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