Figure 6 Experimental results of the normalised transmitted power versus frequency for different values of reverse bias voltage substrate distance from the waveguide vertical walls is
V
R
⫽
0 V (
䊐
); V
R
⫽
10 V (*); V
R
⫽
20 V (
{
); V
R d
10
V
R
⫽
30 V (
E
)
. The
⫽
55 mm.
approaches the wall, the filter’s band-stop performance deteriorates (the band-stop attenuation is reduced).
The simulated shift in the filter’s band-stop frequency due to the mechanical movement of the substrate and the electrical biasing of the varactor diodes is confirmed experimentally in Figure 4.
Again referring to Figure 1, the approximate filter dimensions used in the experiments were a ⫽ 109.5 mm, b ⫽ 56 mm, d mm, d
1 d
6
⫽ 2.2 mm, d
⫽ 2 mm, d
2
7
⫽ 1.6 mm,
⫽ 40 mm, d
3 d
8
⫽ 121 mm,
⫽ 29.5 mm, and d
4 d
9
⫽
5
⫽ 45
183 mm,
⫽ 43 mm. The strip conductor width of the ring element was 1.0 mm and the conductor width of the biasing lines was 0.25 mm. A HP 8722D network analyser was employed to measure the filter response following a thorough calibration. The band-stop attenuation is less than the attenuation obtained in the simulations. This is due to the resistance of the varactor diodes, substrate, and conductor losses.
For the single-diode rectangular ring-element configuration depicted in Figure 1, the band-stop region of the filter tunes over a frequency range of 100 MHz. To further increase the latter range, two varactor diodes can be used in series (see Fig. 5). The effect of the latter is experimentally demonstrated in Figure 6 where the distance d
10 was chosen to correspond to the maximum frequency value that the band-stop region can attain using mechanical tuning, that is, d
10
⫽ a / 2. By increasing the reverse-bias voltage, the band-stop frequency increases by about 200 MHz. The latter tuning range is twice that of the single diode (see Fig. 4). Note that, compared with Figure 4, the attenuation of the band-stop region has been reduced. This is due to the fact that the varactor diodepair has a resistance value which is two times larger (assuming identical diodes) than that of a single diode.
4. CONCLUSION
This paper has proposed a novel, hybrid, mechanically and electrically tunable filter. The tuning mechanisms have been shown to have an additive effect, that is, once the maximum frequency tuning range of the filter has been attained using purely mechanical means, the range is further increased by electrically decreasing the capacitance of the varactor diodes. In addition, the electrical tuning range has been shown to increase by using a diode-pair. Such a configuration, however, also increases the total resistance and consequently the band-stop attenuation of the filter decreases.
Alternatively, the tunable frequency range of the band-stop region can be increased further by using varactors of higher tunability.
Current research work is focused in employing this filter to improve the spectrum efficiency of wireless communications systems.
ACKNOWLEDGMENT
This work was performed under the Radiocommunications
Agency contract AY4115.
REFERENCES
1. C. Tsakonas and C. Mias, Mechanically tunable, microstrip-element waveguide filter. Microwave Opt Technol Lett (accepted).
2. C. Mias, Waveguide and free-space demonstration of tunable frequencyselective surface, Electron Lett 39 (2003), 850 – 852.
3. Ansoft HFSS, ANSOFT, Corporate Headquarters, Four Suite, Pittsburgh, PA, 15219-1119.
© 2004 Wiley Periodicals, Inc.
Kai Chen,
1
1
Qiuqin Sheng,
Institute of Physics
1
Nankai University
Tianjin, 300071, P.R.China
2
Institute of Modern Optics
Nankai University
Tianjin, 300071, P.R.China
and Xiaoyi Dong
2
Received 29 November 2003
ABSTRACT: We present a novel method to fabricate fiber filters based on mechanically induced long-period fiber gratings. Transmission spectra can be tailored by external pressures or by introducing a phase-shift. As an example, the filter was used to flatten the spectral output from an Er-doped superfluorescent fiber source.
© 2004 Wiley Periodicals, Inc. Microwave
Opt Technol Lett 42: 15–17, 2004; Published online in Wiley Inter-
Science (www.interscience.wiley.com). DOI 10.1002/mop.20193
Key words: bandpass; band-rejection; filter; long-period fiber grating
INTRODUCTION
Long-period fiber gratings (LPFGs) with the period of several hundred micrometers have found many applications in telecommunication and sensing-technology fields. They have been introduced as gain equalizers [1], band-rejection filters [2], and sensors
[3]. In an LPFG, mode coupling occurs between the fundamental guided mode and the co-propagating cladding modes, which are determined by the phase-matching condition. Therefore, several lossy peaks can be observed at discrete resonant wavelengths in the transmission spectrum of an LPFG. Most conventional LPFGs are fabricated by UV-light exposure, based on the photosensitivity of the fiber core. Although this method is popular, the spectrum of the grating can hardly be tuned once it has been fabricated, especially when complex spectral shaping is needed. The following alternative methods to fabricate LPFGs have been reported: by
CO
2 laser pulse [4], corrugated structures [5], and grooved plate
[6]. However, these methods still have limited tunability and need
MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 42, No. 1, July 5 2004 15

Figure 1 Experimental setup for the mechanically induced LPFG special precise alignment or accurate etching or complicated mechanical machining.
In this paper, we introduce a simple method to induce a long-period grating mechanically with coiled brass wires in a general single-mode fiber (SMF). The grating is erasable and reconfigurable. Its transmission spectrum can be tuned easily by changing the applied pressures or by introducing different phaseshifts in the grating.
EXPERIMENTAL SETUP
The main mechanical setup is schematically shown in Figure 1. An organic glass plate was wrapped manually by brass wires which have a diameter of 125 ␮ m and were placed periodically on the plate. In our experiment, a period of
⬃
620
␮ m was chosen for the sake of the spectrum range of our broadband source and a total of
47 periods were within the 3-cm-long plate. Pressure was applied on the plate by a spring-loaded apparatus and the fiber (with fiber jacket) is a general G.652 singe-mode fiber with a numerical aperture of 0.139 and a cutoff wavelength of 1308.7 nm. A dummy fiber of the same type was used under the plate to balance the pressure. An Er-doped superfluorescent broadband source was used as the light source and the transmission spectra were monitored by an optical spectrum analyzer (Agilent 86142B).
RESULTS AND DISCUSSION
The growth of the mechanically induced LPFG is illustrated in
Figure 2. The transmission variation near 1530 nm is due to the instability of the broadband source. In the figure,
⌫ is the average pressure on a single wire. The pressure, which is evenly applied to the plate, produces a periodical stain in the fiber via the wires.
Through the photoelastic effect, the stain results in a periodical
Figure 3 Wavelength shift of the resonant peaks of the long-period fiber grating. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.] index variation, which causes mode coupling between the guided mode and the forward-propagating cladding modes, thus forming an LPFG in the fiber. Clearly, higher pressures, and hence higher induced index variation, lead to stronger mode coupling whereby the resonant peaks increase correspondingly, as indicated in Figure
2. Hence, this type of LPFG exhibits the characteristic of tunable peak loss, which can be easily controlled by external pressures. A tuning range of ⬃ 14.5 dB was obtained in our experiment. However, it is expected that the resonant lossy peak will decrease due to mode re-coupling when the pressure is high enough, similar to over-exposure in conventional UV-induced LPFGs. In our experiment, when ⌫ was more than ⬃ 2.56
N , the 1512 nm loss peak began to decrease. Also note that the resonant peak shows a small wavelength shift ( ⬃ 1 nm for a 1512-nm resonant peak and ⬃ 3 nm for a 1458-nm resonant peak) compared with UV-induced LPFGs
[7], as illustrated in Figure 3. In this case, since the Young’s modulus and the refractive indices in the fiber core and the cladding have small difference, the resultant index changes in the core and the cladding are similar, which leads to less wavelength shift according to the phase-matching condition. The typical out-of band loss of the mechanically induced LPFG is less than 2 dB, which can be improved by exerting pretension in the fiber. The
3-dB bandwidth for the 1512-nm peak ranges from 5 to 22 nm.
By leaving the central period of the grating to be ⌳ /4, ⌳ /2 and
3
⌳
/4, where
⌳ is the period of the grating, we obtain
␲
/2,
␲
, and
3
␲
/2 phase-shifted LPFGs, respectively, as shown in Figure 4(a).
With the introduction of phase shift, the band-rejection filters easily become bandpass filters. Obviously, the effective phase shift
Figure 2 Transmission spectra under different pressures.
⌫
1
,
⌫ and
⌫
5 are equivalent to 1.00, 1.61, 2.01, 2.56, and 2.81
N
2
,
⌫
3
,
⌫
, respectively
4
,
Figure 4 Characteristics of phase-shifted LPFG: (a) transmission spectra with phase shifts and without any phase-shift; (b) transmission spectra of
3
␲
/2 phase-shifted LPFG against different pressures.
⌫
1
,
⌫
2
,
⌫
3
, and
⌫
4 are about 3.37, 3.77, 4.72, and 5.68
N , respectively
16 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 42, No. 1, July 5 2004

filter based on corrugated long-period fiber grating, IEEE Photon Technol Lett 13 (2001), 332–334.
6. S. Savin, M.J.F. Digonnet, G.S. Kino, and H.J. Shaw, Tunable mechanically induced long-period fiber gratings, Opt Lett 25 (2000), 710 –712.
7. T.W. MacDougall, S. Pilevar, C.W. Haggans, and M.A. Jackson, Generalized expression for the growth of long period gratings, IEEE Photon
Technol Lett 10 (1998), 1449 –1451.
© 2004 Wiley Periodicals, Inc.
Figure 5 Spectra of Er-doped superfluorescent source with and without flattening is slightly different from the desirable value due to the inaccurate length of the central period. Furthermore, we can tailor the transmission spectrum by changing the applied pressure. Figure 4(b) shows the evolution of a 3 ␲ /2 phase-shifted LPFG under different pressures. This provides another degree of freedom for tailoring the transmission spectra.
Finally, a grating with a period of 630
␮ m and a total of 79 periods was used to flatten the spectral output of an Er-doped superfluorescent fiber source. The result is illustrated in Figure 5.
The variation in the spectral output has been flattened to less than
2 dB over 35 nm, compared to 9.2 dB without the grating. Of course, the amount of flattening can be improved by placing another segment of fiber under the same plate in order to cascade the gratings to flatten the small spectral variations.
CONCLUSION
In this paper, we have presented a novel method to fabricate band-rejection and bandpass filters based on mechanically induced
LPFGs with periodically wrapped wires. This method is simple and reconfigurable and offers great tunability by changing the external pressures or introducing a phase-shift in the grating.
Based on this type of grating, we have successfully flattened an
Er-doped superfluorescent fiber source, obtaining a flatness less than 2 dB over a bandwidth of 35 nm. This type of grating is flexible in spectra shaping and will be useful for special filter designs.
ACKNOWLEDGMENT
This work is supported by the National Natural Science Foundation of China under grant no. 60137010.
REFERENCES
1. A.M. Vengsarkar, J.R. Pedrazzani, J.B. Judkins, P.J. Lemaire, N.S.
Bergano, and C.R. Davidson, Long-period fiber-grating-based gain equalizers, Opt Lett 21 (1996), 336 –338.
2. A.M. Vengsarkar, P.J. Lemaire, J.B. Judkins, V. Bhatia, T. Erdogan, and J.E. Sipe, Long-period fiber gratings as band-rejection filters, J
Lightwave Technol 14 (1996), 58 – 65.
3. V. Bhatia and A.M. Vengsarkar, Optical fiber long-period grating sensors, Opt Lett 21 (1996), 692– 694.
4. X. Li, C. Yue, L. Xia, X. Chen, and X. Xie, Novel technique for long period gratings fabrication using broad spectrum ultraviolet source,
Microwave Opt Technol Lett 33 (2002), 368 –370.
5. Y. Lin and L.A. Wang, A wavelength- and loss-tunable band-rejection
Li Wei,
1
1
Qinghe Mao,
1 and John W. Y. Lit
1,2
Department of Physics and Computer Science
Wilfrid Laurier University
Waterloo, ON, N2L 3C5, Canada
2
Department of Physics
University of Waterloo
Waterloo, ON, N2L 3G1, Canada
Received 16 December 2003
ABSTRACT: In this paper, an analytical analysis is made on a periodic bandpass filter formed by a two-cavity etalon incorporated into a
Sagnac loop interferometer. The general optimum conditions for a quasiflat-top spectrum and a ripple-free group-delay response are derived. Our results show that by properly choosing the reflectance of the mirrors, the filter can be designed with a quasi-flat-top spectral response that is also free from group-delay ripples.
© 2004 Wiley Periodicals, Inc. Microwave
Opt Technol Lett 42: 17–21, 2004; Published online in Wiley Inter-
Science (www.interscience.wiley.com). DOI 10.1002/mop.20194
Key words: optical filters; Sagnac loop; wavelength division multiplexing (WDM); group delay
INTRODUCTION
Optical filters are essential components in dense wavelengthdivision-multiplexing (DWDM) systems and networks for realizing various fundamental network functions, including add-drop, multiplexing/demultiplexing, and interleaving. For such applications, the filters will perform best if they have flat-top spectral responses. Such flat-top filters can be made with various elements, including fiber Bragg gratings (FBG) [1, 2], planar lightwave circuits (PLCs) [3, 4], optical-fiber delay lines [5], and various interferometers based on bulk optics [6, 7]. Recently, a ripple-free optical filter using a Michelson interferometer (MI) with a Gires–
Tournois resonator (GTR) inserted into one arm was proposed and theoretically analyzed [6]. The performance of the filters was then improved when both arms of the interferometers had a GTR built into them [7]. However, as bulk-optic elements are used, the light beams have to be collimated and the system has high losses. On the other hand, the Sagnac loop interferometer (SLI) in fiber form is much more robust by comparison. It has the advantages of low loss and low cost. More significantly, it is inherently compatible with fiber systems. A periodic filter [8, 9] formed by an SLI with an
FBG in the fiber loop has been proposed and demonstrated. However, because only one reflector (the FBG) is used, the spectral response of the filter has a Lorentzianlike shape. In order to achieve flat-top spectral response, more reflectors may have to be used in a Sagnac loop. Thus, it is very important to investigate and optimize the functions of an SLI with multiple reflectors with
MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 42, No. 1, July 5 2004 17