All-fiber polarization interference filters based on
45° ° ° °-tilted fiber gratings
Zhijun Yan1, *, Chengbo Mou1, Hushan Wang1, 2, Kaiming Zhou1, Yishan Wang2, Wei Zhao2, Lin Zhang1
1Photonics Research Group, Aston University, Birmingham, UK, B4 7ET
2State Key Laboratory of Transient Optics and Photonics, Xi’an Institute of Optics and Precision Mechanics, Chinese
Academy of Sciences, Xi’an 710119, China
*Corresponding author: email@example.com
Received Month X, XXXX; revised Month X, XXXX; accepted Month X,
XXXX; posted Month X, XXXX (Doc. ID XXXXX); published Month X, XXXX
We report all-fiber polarization interference filters, known as Lyot and Lyot-İhman filters, based on alternative
concatenation of UV-inscribed fiber gratings with structure tilted at 45° and PM fiber cavities. Such filters generate comb-
like transmission of linear polarization output. The free spectral range (FSR) of a single-stage (Lyot) filter is PM fiber cavity
length dependent, as a 20 cm long cavity showed a 26.6nm FSR while the 40 cm one exhibited a 14.8nm FSR. Furthermore,
we have theoretically and experimentally demonstrated all-fiber 2-stage and 3-stage Lyot-İhman filters, giving more
freedom in tailoring the transmission characteristics.
OCIS codes: 060.2340, 230.7408, 350.2460, 050.2770.
The Lyot filter is a kind of polarization interference filter,
which was invented by Bernard Lyot in 1933 . It is
formed by placing a birefringence medium (BM) between
two parallel polarizers whose axes are aligned at 45° with
respect to the axis of the BM. Lyot filters can be used as
comb transmission filters, due to the property of
continuous wavelength-dependent transmission bands in
the spectra. Later, İhman demonstrated a new
monochromator by connecting several Lyot filters in
series, which was named as Lyot–İhman filter . The
Lyot filters were widely applied in spectral imaging [3, 4],
communication , and laser [6-7] systems. Due to
unavailability of in-fiber components, the majority
reported Lyot and Lyot-İhman filters were constructed
using bulk polarizers with a polarization maintaining
(PM) fiber, inducing not just high insertion loss but also
the chunky structure which is not compatible with all-
fibre systems. Tilted fiber gratings (TFGs) were firstly
reported by Meltz et al in 1990 . Owing to their unique
polarization function, TFGs have been applied as
polarimeter, PDL equalizer and so on [9-10]. Recently, we
have demonstrated in-fiber polarizers with high
polarization extinction ratio (PER) based on 45°-TFGs
UV-inscribed in single mode telecom and photosensitive
fibers [11, 12]. We have also demonstrated that the 45°-
TFGs can be UV-inscribed into PM fibers along the fast-
or slow-axis, leading to linear polarization light output. In
this letter, we propose and demonstrate all-fiber Lyot and
Lyot-İhman filters that are implemented by alternatively
concatenating 45°-TFGs and PM fiber cavities with fiber
axis at 45° to each other. The bandwidth of the filter is
easily controlled by changing the length of PM fiber
cavity, while the FSR of transmission bands can be freely
tailored by using multi-stage Lyot filters.
The configuration of an all-fiber Lyot filter using two
45°-TFGs is illustrated in Fig 1 (a). In this structure, the
two 45°-TFGs created in PM fiber along the fast axis are
used as linear polarizers, and a section of PM fiber is used
as a BM with its fast axis aligned 45° to that of the two
Fig. 1 (a) The configuration of an all-fiber Lyot filter using two
45°-TFGs and a PM fiber cavity; (b) the configuration of an all-
fiber 3-stage Lyot–İhman filter using four 45°-TFGs with three
PM fiber cavities at a ratio 4:2:1.
The working principle of this all-fiber Lyot filter is as
follows: the light passed the first 45°-TFG is linearly
polarized, which then enters the PM fiber at 45° direction
and is resolved into two beams with equal intensity
travelling along the fast- and slow-axis of the PM fiber,
respectively. Due to the birefringence of the PM fiber,
there is a relative phase difference between these two
beams, thus when they arrive at the second 45°-TFG, the
combined beam will generate interference output of linear
polarization state. Fig. 1b shows a 3-stage all-fiber Lyot–
İhman filter, which can be formed by concatenating four
45°-TFGs spaced by three PM fibers with a length ratio
4:2:1 and all aligned at 45° to the fast axis of the PM fibers
that are hosting 45°-TFGs. In this 3-stage Lyot-İhman
filter, only the light at the wavelengths in phase (i.e. the
phase difference is multiple of 2π) will be constructive and
pass the filter. The final output spectrum thus consists of
a serial of transmission bands. By applying the transfer
matrix method, the m-stage Lyot filter can be described
Where, for simplifying the calculation, we defined the fast
axis of PM fiber that is hosting 45°-TFG as x-axis, and the
fast axis of the PM fiber cavity is aligned at 45° with
respect to the x-axis. In Equation (1), ∆φm is the relative
phase difference induced by mth section BM. It is defined
Where, Lm is the length of PM fiber of mth BM; ∆n is the
birefringence of PM fiber; λ is the working wavelength.
The normalized transmittance of m-stage Lyot filter is
given by :
Also, the FSR of Lyot filter can be obtained from Equation
(2) [13, 14], as:
Fig. 2 The spectral polarization extinction ratio over wavelength
range from 1530nm to 1608nm. Inset: microscopic image of 45°-
TFG structure in a PM fiber.
All 45°-TFGs were inscribed in PM1550 fiber (Corning)
along the fast axis by using the scanning phase-mask
technique and a 244nm UV source from a CW frequency
doubled Ar⁺ laser (Coherent Sabre Fred®). The phase-
mask (Ibsen) has a uniform pitch of 1830nm and 33.7°
tilted angle with respect to the fiber axis. The tilted pitch
pattern in the phase-mask glass plate is 50mm long, thus
giving an effective grating length around 49mm in the
fiber core. The PER values of the 45°-TFGs were
measured by an Optical Vector Analyzer system
(OVA2000) from Luna Technologies' CTO. Fig. 2 shows
the PER distribution of one of the UV-inscribed 45°-TFGs
from 1525nm to 1608nm. The maximum PER is 33dB at
1530nm and dropped to 29dB at 1608nm. The
polarization distribution figure is shown in Fig. 3, which
was measured using the method presented in our
previous paper [9, 10]. The near-perfect figure ‘8’ shape
indicates the 45°-TFG in PM1550 fiber can act as a highly
Figure 3 The polarization distribution of one of the 45°-TFGs UV-
inscribed in PM fiber.
We first constructed and evaluated all-fiber single-stage
Lyot filters with different cavity lengths. The PM fiber
sections of 20, 40 and 80cm were spliced between the two
45°-TFGs in turn forming three Lyot filter structures. The
two ends of the PM cavity fiber were spliced with its fast
axis aligned at 45° to the fast axis of the two 45°-TFGs.
Fig. 4a shows the measured spectra of the three Lyot
filters. It can be clearly seen from the figure that the
output from the 45°-TFG based Lyot filter exhibits comb-
like transmission and the bandwidth and FSR are cavity
length dependent. The FSRs for 20, 40 and 80cm PM fiber
cavities are 26.6nm, 14.9 nm, and 6.9 nm, respectively,
showing the FSR is inversely proportional to the length of
PM fiber cavity. The longer the cavity length is the
smaller FSR. All the maximum transmissions occur when
the relative phase difference is equal to 2mπ, (m = 0, 1, 2,
3…), and in contrast, the minima occur when the phase
difference is equal to (2m+1) π.
Fig. 4 (a) Measured transmission spectra of three 45°-TFG based
all-fiber Lyot filters with 20cm, 40cm and 80cm cavity length; (b)
Experimentally measured and theoretically calculated the
relationship between FSR of the filter and the length of PM fiber
We also experimentally measured the FSR of seven Lyot
filters with cavity lengths of 10, 20, 30 40, 60 and 80cm
and the results are plotted in Fig. 4b, which are in
excellent agreement with the theoretically calculated
ones, and the whole curve shows the inversely
proportional relationship between the FSR and Lyot filter
cavity length. Note, in the calculation, the birefringence of
the PM fiber was estimated by inscribing a fiber Bragg
grating (FBG) with known period in the PM fiber and the
birefringence value of the PM fiber was estimated around
Fig. 5 The simulated (solid) and experimentally measured (dash)
comb-like transmission spectra of (a) 2-stage Lyot filter with PM
fiber cavity length ratio 1:2 (20cm and 40cm) and (b) 3-stage Lyot
filter with ratio 1:2:4 (20cm, 40cm and 80cm).
We further experimentally constructed a 2- and 3-stage
Lyot-İhman filters by concatenating three and four 45°-
TFGs with PM fiber cavity length ratios of 1:2 (20 and
40cm) and 1:2:4 (20, 40 and 80cm), respectively. The
transmission spectra of the 2- and 3-stage Lyot-İhman
filters are shown in Fig. 5a and b. It can be seen clearly
from the figure that the transmission bands of the Lyot-
İhman filters are generated from coupled cavities, i.e. the
transmission maxima are only occur at those wavelengths
in phase for each individual cavities and the other non-
phase matched wavelengths are suppressed. We also
simulated the transmission spectra of the 2- and 3-stage
Lyot-İhman filter and the results are also plotted in Fig.
4. It can be seen that the experimental results are in very
good agreement with the simulation ones. The strength of
each peak is not quite the same for the 3-stage filter, as
shown in Fig. 5b; this is because the length ratio of PM
fiber cavity may not be exactly at 1:2:4. We can also find
that the bandwidth of the multi-stage Lyot-İhman is
determined by the length of the longest while the FSR is
by the shortest cavity in multi stage filters.
All-fiber Lyot and Lyot-İhman filters have been
demonstrated by concatenating 45°-TFGs and PM fibers.
The 45°-TFGs were UV-inscribed along the fast axis of the
PM fiber and then spliced with the PM fiber cavities with
designed length ratio and at 45° to their principal axes.
Such polarization interference filters generate comb-like
transmission spectrum, showing the FSR can be easily
designed by altering the cavity length. The multi-stage
filters offer another dimension to tailor the comb-like
transmission spectral response with desirable FSR. The
filters all give linearly polarization output, which will
make them good in-fiber polarization filters for
applications in fiber laser, spectral imaging, optical
communication and sensing measurements and systems.
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