notch filter with independent ... - OSA Publishing

Optical bandpass/notch filter with independent tuning of wavelength and bandwidth based on a blazed diffraction grating Bo Dai,1,2 Dong Wang,1 Chunxian Tao,1 Ruijin Hong,1 Dawei Zhang,1 Songlin Zhuang,1 and Xu Wang2,* 1 Engineering Research Center of Optical Instrument and System, the Ministry of Education, Shanghai Key Laboratory of Modern Optical System, University of Shanghai for Science and Technology, Shanghai, 200093, China 2 The Institute of Photonics and Quantum Sciences, School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, UK * [email protected]

Abstract: We propose a multifunctional optical filter based on a blazed diffraction grating. The optical filter can function as a bandpass filter or a notch filter. A theoretical model of the filter is built for analysis. Both bandwidth and wavelength of the filter can be independently and continuously tuned. In the experimental demonstration, the wavelength can be linearly tuned within the entire C-band and partial L-band. The bandwidths of the filter can be tuned from 1.3 to 6.4 nm (–3 dB bandwidth) and from 2.4 to 11.3 nm (–10 dB bandwidth) for bandpass function and from 6.9 to 11.9 nm (–3 dB bandwidth) and from 5.1 to 8.8 nm (–10 dB bandwidth) for band-stop function, respectively. The extinction ratio of more than 35 dB is achieved. ©2014 Optical Society of America OCIS codes: (230.7408) Wavelength filtering devices; (230.1950) Diffraction gratings.

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#216798 - $15.00 USD Received 10 Jul 2014; revised 5 Aug 2014; accepted 6 Aug 2014; published 14 Aug 2014 (C) 2014 OSA 25 August 2014 | Vol. 22, No. 17 | DOI:10.1364/OE.22.020284 | OPTICS EXPRESS 20284

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1. Introduction Optical filters are essential devices to selectively manipulate the signals in the different spectral bands, which are widely used in the optical communication systems and optical signal processing applications. In the spectrally-multiplexing optical communications, such as wavelength-division multiplexing (WDM) and orthogonal frequency division multiplexing (OFDM) systems, spectrally flexible elastic optical networking is preferred for a high spectral efficiency and a power-efficient manner [1–4]. Spectrally flexible elastic optical networking requires the provision of dynamic spectral-band allocation. Therefore, it increases the importance on tunable optical filtering. Optical filters should provide independent and continuous tuning of both wavelength and bandwidth to cater to the flexible spectral arrangement [5, 6]. In the optical signal processing, the spectral shift of the optical signal occurs in the many nonlinear processing, such as self-phase modulation (SPM), cross-gain modulation (XGM) and four-wave mixing (FWM). Subsequently, optical filtering is an essential technique to filter out or suppress the spectral shifted components after nonlinear optical processing [7–9]. The requirement of the bandwidth and center wavelength of filtering is crucial for signal quality. Thus, a multifunctional optical filter with the capability of bandwidth and wavelength tuning is desired. A lot of optical components have been developed into filters, including fiber Bragg grating (FBG), virtual image phased array (VIPA), microring resonator (MRR) and microelectromechanical system (MEMS) [10–15]. Nevertheless, only few bandpass filtering methods have been proposed for independent tuning of both wavelength and bandwidth. In [16], an optical bandpass filter has been proposed, which was based on a double-pass monochromator configuration and a variable-aperture MEMS reflector with two blocking micromirrors. The –1-dB bandwidth of the filter was tunable from 3.65 to 6.35 nm and the wavelength could be controlled within a 30 nm spectrum. Another MEMS based optical bandpass filter for –3-dB bandwidth tuning of 0.3 to 1.5 nm and entire C-band wavelength tuning has been demonstrated by dispersing an input light on a MEMS spatial light modulator [17]. An optical bandpass filter based on two microring in a Mach-Zehnder interferometer has been presented in [18]. By thermally controlling the resonance offset between the two microring resonators, the bandwidth and wavelength could be tuned from 0.46 to 0.88 nm and from 1550 to 1554 nm, respectively. In this paper, an independent wavelength and bandwidth tunable optical bandpass and notch filter is proposed. The filter is mainly based on a blazed diffraction grating and a reflection plate of specific pattern. The functions of the filter can be shifted between bandpass and band-stop. The paper is organized as follows. In Section 2, a mathematical model is built to explain the operation principle of the filter. In Section 3, the filtering characteristics are experimentally demonstrated. Finally, concluding remarks are made in Section 4. 2. Operation principle Figure 1 shows the schematic diagram of the proposed filter. A light is fed into Port 1 of an optical circulator. Port 2 of the circulator is connected to a beam collimator. The collimated beam passes through a beam expander, which consists of a pair of achromatic doublets and a biconcave lens in between. After the beam expander, the radius of the beam size is adjusted to Win (intensity is 1/e2 times the maximum). Then, the collimated beam is diffracted by a blazed


diffraction grating. The blazed diffraction grating dispersed the beams of different wavelengths in the different positions of x-dimension. A spherical biconvex lens with the focus length of f is used to focus all dispersed beams onto the back focal plane. The E-field of the light on the back focal plane can be expressed as follows [19].

  x − αω 2  E ( x, λ ) ∝ Ein ( λ ) exp −      Wo  

(1)

where

α=

λ2 f

2π cd cos (θ d )

Wo =

cos (θin ) f λ cos (θ d ) π Win

1

ω = 2π c 

λ

−

1  λ0 

(2)

(3)

(4)

and Ein is the amplitude of the input light, c is the light speed, d is the grating period, θin is the incident angle of the collimated light into the grating, θd is the diffracted angle of the grating, f is the focal length of the lens, and λ0 is the wavelength of the light with the peak intensity, whose ray path is perpendicular to the lens and reflection plate.

Fig. 1. Setup of the proposed optical filter.

On the back focal plane of the spherical biconvex lens, a reflection plate is placed. The reflection plate is made of a colourless optical glass as substrate. One side of the reflection plate is coated with a specific pattern for reflection and can be divided into two zones as shown in Fig. 2. In the middle of Zone I, a narrow slit is coated with aluminium for reflection, while in Zone II the whole zone except the middle slit is coated with aluminium. Partial dispersed beams pass through the uncoated area of the reflection plate, and the rest of the dispersed beams are reflected from the coated area back into Port 2 of the circulator through the same optical path and output from Port 3.


Bandpass

I(λ)

Output spectrum reflected from Zone I

λ

Reflection plate

I(λ)

Zone I

Input spectrum

Zone II

λ I(λ)

Output spectrum reflected from Zone II

Bandpass

Band-stop

Change wavelengths

Change functions Band-stop

y x

λ

Fig. 2. Schematic diagram of the reflection plate.

Zone I and Zone II are for bandpass filtering and band-stop filtering respectively and can be shifted by changing the position of y-dimension. Furthermore, since the beams of different wavelengths are dispersed along x-dimension, the wavelength of the filter can be adjusted by moving the reflection plate along x-dimension. The output E-field can be written as follows.  Ein ( λ )   L /2+Δx −αω   L /2 −Δx +αω   2 erf  Wo  + erf  Wo   Eo ( λ ) ∝   L /2 −Δx +αω   x −αω   Ein2( λ ) 1− erf  L /2 +Δ  − erf   Wo Wo        

Bandpass

(5) Band − stop

where erf(·) is the expression of error function, L is the width of the narrow slit, and Δx is the moving position of the reflection plate along x-dimension. In addition, according to the calculation based on Eq. (5), the input beam size, Win, has the influence over the bandwidth of the filter, which will be demonstrated in the following section. The group delay is a very important factor of many filtering applications. In the filter, the group delay occurs between the diffraction grating and the lens, neglecting the thickness of the lens. The group delay can be estimated using the following equation.

τ=

(

c cos sin

2f

−1

( λ / d − sin (θ ) ) − sin ( λ −1

in

0

/ d − sin (θin ) )

)

(6)

3. Experimental demonstration

3.1 Experimental setup In the experiment, the center wavelength of the light is at 1557 nm. The beam from the collimator has the beam size of 3.6 mm. The beam expander has the expansion ratio from 3x to 15x, which can adjust the input beam size from 1.2 to 0.24 mm. The groove density of the blazed diffraction grating is 600 lines/mm. The incident angle on the grating is about 35°. The focal length of spherical biconvex lens is 100 mm. The fabricated reflection plate is shown in Fig. 3. The width of the middle slit is 70 μm.


Fig. 3. The reflection plate used in the experiment. Microscope magnification: 50x.

3.2 Bandpass filter To demonstrate the bandpass filter, the reflection plate is moved downwards to make the dispersed beams focused on Zone I. Figure 4 illustrates the output of the filter when the input beam size is 0.36 and 0.9 mm respectively. The features of center wavelength tuning and bandwidth tuning can be easily observed. The insertion loss of the filter is about 4 dB. The extinction ratio of the filter is better than 35 dB, which can be further improved by using high reflection coating for C- and L-band light.

Fig. 4. The output spectra of the bandpass filter for input beam size of (a) 0.36 and (b) 0.9 mm. Dashed line: the spectrum of the input signal. Solid line: transmission characteristics of the filter.

The center wavelength tuning is determined by the spectral dispersion of the optical system and the spectral dispersion is closely related to the focal length of the spherical biconvex lens. Figure 5 shows the calculated spectral dispersion when the focal length of the biconvex lens is changed and the other parameters are as described in Section 3.1. The short focal length results in the low dispersion. In the experiment, the focal length of the lens is 100 mm to achieve enough dispersion for easy wavelength tuning. The center wavelength of the filter is tuned by adjusting the position of the reflection plate along the x-dimension. The relationship between the position of the reflection plate and the center wavelength is depicted in Fig. 6. The solid black line is calculated by using Eq. (5). The experimental results fit the calculated results very well. The tuning of the center wavelength is linear and continuous with the position change of the reflection plate. The center wavelength is tuned from 1532 to 1582 nm, covering the whole C-band and partial L-band. The tuning range of the wavelength depends on the free-spectral range (FSR) of the first order of the blazed diffraction grating.


Fig. 5. The spectral dispersion of the optical system with different focal length of the biconvex lens.

Fig. 6. The measured results of the relationship between the position of the reflection plate and the center wavelength of the filter when the focal length of the biconvex lens is 100 mm.

The bandwidth of the filter is determined by the spectral resolution and spectral dispersion of the grating and the slit width of the reflection plate. When the spectral dispersion and the slit width are fixed, the bandwidth is inversely proportional to the number of the grating lines illuminated by the incident beam. Thus, the incident beam size affects the bandwidth of the filter. Figure 7(a) shows the bandwidth tuning versus the change of the incident beam size on the blazed diffraction grating. The blue curve is the calculated results and the red dots represent the experimental results. The experimental results match the theoretical ones well. The bandwidth of the filter shrinks with the increase of the incident beam size on the blazed diffraction grating. The –3 dB and –10 dB bandwidths can be tuned from 1.3 to 6.4 nm and from 2.4 to 11.3 nm, respectively. The spectra of the bandpass filter centered at 1557 nm for incident beam size of 0.36 and 0.9 mm are shown in Fig. 7(b). It is obvious that the bandwidth of the filter for small incident beam size is wide. The group delay response of the filter is depicted in Fig. 8. The group delay over 50 nm bandwidth is less than 150 fs. The light with the wavelength far away from that of the peak intensity has relatively large time delay. The focal length of the lens affects the group delay response and the long focal length leads to large group delay, because the long focal length aggravates the difference of the travelling distance of the light with different wavelengths.


Fig. 7. (a) The bandwidth tuning of the bandpass filter. (b) Comparison of –3 dB bandwidth for incident beam size of 0.36 and 0.9 mm.

Fig. 8. The group delay response of the filter

3.3 Notch filter If the reflection plate is moved upwards to make the dispersed beams focused on the Zone II, the filter functions as a notch filter. Figure 9 shows the output spectra of the notch filter when the incident beam sizes on the grating are 0.48 and 0.9 mm. The narrow uncoated slit rejects the reflection of the dispersed light and leads to a deep dip in the spectra. The extinction ratio is more than 38 dB. By linearly and continuously adjusting the position of the reflection plate along x-dimension, the center wavelength is shifted smoothly. The shifting of the wavelength corresponding to the position change of the reflection plate is shown in Fig. 6. The tuning range of the wavelength is about 50 nm.

Fig. 9. The output spectra of the notch filter for input beam size of (a) 0.48 and (b) 0.9 mm. Dashed line: the spectrum of the input signal. Solid line: transmission characteristics of the filter.


The –3 dB bandwidth tuning of the notch filter is illustrated in Fig. 10(a). By adjusting the expansion ratio of the beam expander, the incident beam size on the grating, Win, decreases and the bandwidth of the notch filter become wide. Especially, when the incident beam size is smaller than 0.7 mm, a rapid change of the bandwidth can be observed. The spectra of the notch filter for the incident beam size of 0.48 and 0.9 mm are depicted in Fig. 10(b). The –3 dB bandwidths are 8.8 and 7.3 nm respectively. The measured –3 dB and –10 dB bandwidth tuning ranges of the notch filter are from 6.9 to 11.9 nm and from 5.1 to 8.8 nm, respectively.

Fig. 10. (a) The bandwidth tuning of the notch filter. (b) Comparison of –3 dB bandwidth for incident beam size of 0.48 and 0.9 mm.

4. Conclusions

A wavelength- and bandwidth-tunable optical filter is proposed and demonstrated. By changing the zones of the reflection plate, the filter functions as a bandpass filter or a notch filter. Due to the large FSR of the blazed diffraction grating, the wavelength tuning range of the bandpass and notch filter covers the entire C-band and partial L-band. The bandwidth of the filter can be easily controlled by adjusting the expansion ratio of the beam expander. The measured –3 dB (–10 dB) bandwidth tuning ranges for the bandpass filter and the notch filter are from 1.3 to 6.4 nm (from 2.4 to 11.3 nm) and from 6.9 to 11.9 nm (from 5.1 to 8.8 nm) respectively. An extinction ratio of more than 35 dB is achieved, which is able to suppress the unwanted signals efficiently. The multifunction of the filtering, flexible wavelength and bandwidth tunability and the good extinction ratio makes the proposed scheme a promising candidate to serve in the next-generation optical communications and in the various optical signal processing applications. Acknowledgments

This work is partly supported by National Natural Science Foundation of China (61378060, 61205156, 11105149), National Science Instrument Important Project (2011YQ14014704, 2013YQ16043903), the Shanghai Dawn Project of Shanghai Education Commission (11SG44) and Pujiang Project of Shanghai Science and Technology Commission (14PJ1406900). Bo Dai acknowledges the Scottish Universities Physics Alliance (SUPA) Industrial Placement Scheme.
