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We design and demonstrate a birefringent Michelson interferometer based interleaver with ultra-low dispersion and low cost. The interleaver consists of polarizing beam splitters (PBS’s) and quarter-wave plates and half-wave plates. The PBS’s based Michelson interferometers provide the optical path difference for interference between the two orthogonal polarization components and the half-wave plates provide the birefringent needed to minimize ripple of output. The designed interleaver with two-stage interferometer in a 50 GHz channel spacing application exhibits a 0.5 dB passband and a 25 dB stopband both 27GHz; a channel isolation higher than 35 dB and chromatic dispersion less than ±5 ps/nm within 0.5 dB passband; 1.3 dB insertion loss and 0.3 dB PDL; 0.04GHz/°C thermal stability. Since all of the optical components can be optically bonded together, the device is robust and easy to be aligned, which reduces labor cost.
©2011 Optical Society of America
1. Introduction
The optical interleaver is one of the core devices in optical communication systems [1,2]. It is used to combine two sets of dense wavelength-division multiplexing (DWDM) signal channels (odd and even channels) into a composite signal stream in an interleaving way, and also can be used in a reverse direction. Some important features of an interleaver include wide and flat passband, low dispersion, low insertion loss, low PDL, high channel isolation and very low thermal drift. There are several kinds of interleavers available in the market today, including birefringent interleaver [1], Mach-Zehnder (M-Z) interferometer [3–6], Gires-Tournois Interferometer (GTI) [7–14], etc. Each interleaver technique has its advantages and disadvantages. A birefringent interleaver is composed of birefringent filters which are based on the interference of polarized light. It requires phase retardation between the components of the light polarized parallel to the fast and slow axes of the crystal when the light passes through it. The free-spectral range (FSR) of the narrow spacing interleaver is short, which requires large phase retardation. Due to small birefrigent of crystal it needs long crystal to cause large phase retardation. Accordingly, as a practical matter, birefringent interleavers have almost disappeared in the current market because of the high cost of birefrigent material. A flat-top Fourier filter (F3T) interleaver [4] is an unbalanced Mach-Zehnder interferometer. It consists of three cascaded couplers of different coupling ratios linked by two differential delays. F3T interleaver can be made as an all-fiber device using fusion technology. It has been shown to have wide passband and low insertion loss, low PDL and low dispersion. However, due to the temperature sensitivity of the refractive index of the fiber, the device must be actively temperature controlled, usually be housed in a heated box in order to maintain a constant performance over the operation temperature range, which adds the complexity to the device. A Michelson-Gires-Tournois-Interferometer (MGTI) based interleaver consists of a typical Michelson Interferometer (MI) in which one (or two) of its reflecting mirrors is a Gires-Tournois-Etalon (GTE). Michelson interferometers provide the optical path difference for interference. As a phase dispersive element the GTE allows the MGTI interleaver to have wider passband and stopband than other interleavers. However, the GTE also causes the MGTI interleaver to have the largest chromatic dispersion. Even with CD compensation the MGTI interleaver still has larger CD [15]. In some cases, an additional optical circulator is often needed to separate the input and output signals for MGTI interleavers, which raises cost.
This paper presents a novel birefringent Michelson interferometer based interleaver that consists of polarizing beam splitters (PBS’s), quarter-wave plates (QWP’s) and half-wave plates (HWP’s). The main advantage of this novel interleaver over that based on MGTI is that it eliminates large CD caused by GTE. In addition, the thermal stability of the novel interleaver is much better than that of a fiber M-Z interleaver. Compared to other interleavers this novel interleaver has several unique features including: robust structure; small size; inexpensive materials and low labor cost, which increase competition of the device in market.
2. Design
Figure 1 is a schematic drawing of a birefringent Michelson interferometer based interleaver with two-stage interferometer. It consists of two PBS’s, four HWP’s, two QWP’s and three quarter-wave mirrors (QWM’s) which are quarter-wave plates with high-reflective coating on one side of the plate. All waveplates are zero-order and very thin (HWP ≈0.09mm and QWP (QWM) ≈0.045mm) and they are optically contact on PBS’s (see Fig. 1). A Michelson interferometer includes a PBS, a mirror and a QWM. The mirror is also in optically contact with the PBS through two spacers that are made of ultralow thermal expansion material. In this paper the azimuth angle of wave plate is defined as an angle between slow axis of the wave plate and horizontal (paper) plane. The azimuth angles of QWP and QWM are 45°. The c-axis’s of walk-off crystals YVO4-1 and YVO4-2 are oriented parallel and perpendicular to the plane of propagation, respectively. The azimuth angles of HWP1 and HWP2 are −67.5° and 67.5°, respectively. In this design we therefore only need determine the azimuth angles θ of HWP3 and α of HWP4 according to requirement of minimizing ripples of flat-top passband.
Fig. 1 The scheme (top view) of. the birefringent Michelson interferometer based interleaver with two-stage interferometer. HWP–half-wave plate; QWP—quarter-wave plate; QWM—quarter-wave mirror; PBS–polarizing beam splitter. E–signal-E; O–signal-O.
Figure 1 illustrates the device operating as a deinterleaver; it is a three port device that receives a set of optical signals including even and odd channels through one input port and provides a set of odd channel signals through one output port and a set of even channel signals through another output port. Walk-off crystal YVO4-1 separates the signals from a collimator (not shown in Fig. 1) into vertically polarized (ordinary wave or o-wave) signal-O and horizontally polarized (extraordinary wave or e-wave) signal-E in the horizontal plane. HWP1 rotated the e-wave by 45° and HWP2 rotates the o-wave by −45°. After passing the HWP’s, the polarizations of signal-O and signal-E are parallel with each other and their azimuth angels are both 45°. In the description below we discuss only the propagating process of signal-O as illustrative of both signal-E and signal-O.
Referring to Fig. 1 and Fig. 2 , PBS1 separates the signal-O into s-wave (signal-1) and p-wave (signal-2) with equal amplitudes. In our interleaver as shown in Fig. 1, the o-wave corresponds to s-wave while e-wave corresponds to p-wave. The signal-1(s-wave) is reflected by PBS1 to QWM1, which reflects the signal-1 back to PBS1 and transforms its status of polarization (SOP) from s-wave to p-wave (see Fig. 2). Then the signal-1 (p-wave) passes through PBS1 and is directed to HWP3. The signal-2 (p-wave) passes PBS1 and QWP1, and then is reflected by a mirror-1 to pass QWP1 again, which transforms SOP of the signal-2 from p-wave to s-wave (see Fig. 1 and Fig. 2). Now the signal-2 (s-wave) is reflected by PBS1 forward to HWP3. The signal-1 and the signal-2 then combine into an elliptically polarized optical signal, because there is a path length difference 2d (where “d” = the one-way path length difference of the two arms of the first Michelson interferometer) between the signal-1 and −2. HWP3 rotates the SOP of this elliptically polarized optical signal that passes through HWP3. PBS2 separates this combined optical signal into s-wave (signal-3) and p-wave (signal-4) also. PBS2 reflects the signal-3 (s-wave) to pass QWP2, and then it is reflected by a mirror-2 to pass QWP2 again, which transforms SOP of the signal-3 from s-wave to p-wave. Then the signal-3(p-wave) passes through PBS2 and is directed to HWP4. The signal-4 (p-wave) passes PBS2 to QWM2, which reflects the signal-4 back to PBS2 and transforms its SOP from p-wave to s-wave. The signal-4 (s-wave) is then reflected by PBS2 to HWP4. There is a path length difference of 4d (path length difference of the second Michelson interferometer) between the signal-3 and −4. The SOP of the signal-3 and −4 combined optical signals (signal-A) is rotated by HWP4 and going to the walk-off crystal YVO4-2. The crystal YVO4-2 separates signal-A (in vertical plane) into o-wave including odd channel signals (odd signal-O) and e-wave including even channel signals (even signal-O) (see Fig. 3a ). Similar to the signal-O, the signal-E is separated into the odd signal-E and the even signal-E by YVO4-2 crystal. Using another walk-off crystal similar with YVO4-1 (not shown in Fig. 3a) to combine the odd signal-O with the odd signal-E, and the even signal-O with the even signal-E, and subsequently the device deinterleaves odd and even channels to different output ports from input port for single-pass structure.
Fig. 2 The light signal propagating path and its status of polarization in the device: arrows with filled circles, perpendicular bars, diagonal bars, and ellipses represent vertically, horizontally, 45°, and elliptically polarized optical signals, respectively.
In order to determine the azimuth angles of HWP3 and HWP4, θ and α, respectively, Jones calculus [16] is used to calculate the single-pass spectral transmittance of the interleaver at one of the output ports (odd channel) and the result is
whereand λ is wavelength.Fourier series of a periodic rectangular spectral transmittance can be expressed as [17,18]
where Fourier series coefficients a0 = 0.5, an = sinc(n/2) [17]. Comparing Eq. (1) with Eq. (3), we take a1 = 0.636 and a3 = −0.212 as zero-order approximate values of A and B in Eq. (1), respectively. We then calculate insertion loss IL(λ) = 10log(I(λ)) with d = 1.499 mm for 50GHz interleaver. The solid curve in Fig. 4(a) represents the calculated result and shows that it is not a desired interleaver due to its small isolation and large ripple. According to minimizing ripple we optimized parameters A and B, and found that the optimum value of A and B are 0.575 and −0.083, respectively. The dotted curve in Fig. 4(a) shows a much-improved result based on the optimum values of A and B, but the isolation (−20 dB) is still not large enough to eliminate crosstalk. There are two methods to increase isolation: one is to increase the number of stage of interferometer, this would increase the size and cost. Anotheris the double-pass method: It feeds the output of an interleaver back to itself. QWM3 (see Fig. 1 and Fig. 3b) reflects four outputs: the odd signal-O, the odd signal-E, the even signal-O and the even signal-E from the single-pass device back to it and also changes their polarizations by 90° (see Fig. 3b). The backward waves and forward waves are in different positions in the vertical plane: the backward even channel signals are on top; the forward waves are in the middle and the backward odd channel signals are at the bottom (see Fig. 3b). As such, the positions of the double-pass output ports in YVO4-1 crystal are in the same order: even channel output port on top; input port in the middle and odd channel output port at the bottom. Figure 4(b) shows the spectral transmittance of a double-pass device with the optimum values of A and B. Using the optimum values of A and B we solved equations for α and θ,and find 4 sets of values of (α,θ): (81.5°, −61.5°); (−54°,29°); (−35.9°,61.5°) and (35.9°,-61.5°). In this design we chose α = 81.5° and θ = −61.5°. The calculated results indicate that 0.5 dB passband and 25 dB stopband are both greater than 28 GHz and the channel isolation is higher than 40 dB for a 50GHz interleaver with two-stage interferometer.Fig. 4 Simulated insertion loss (dB) of a novel interleaver as function of wavelength (nm): (a) single-pass structure, the solid curve —using approximate values of A and B in Eq. (1) and the dotted curve—using the optimum values of A and B in Eq. (1). (b) double-pass structure with the optimum values of A and B, including even and odd channels.
Chromatic dispersion is caused by the material and the configuration of the device. In this design, there is no GTI, the main dispersion source is the birefringence of the material, and since the total length of all wave plates is less than 1mm, the material causes a small CD. To achieve zero CD design, compensation is still necessary. The double-pass method also can be used to compensate CD [19] without adding components. In this method the interleaver is designed in such a way that the forward and the backward signals have the opposite CD slopes. As a result, the overall CD is cancelled. In this design the polarizations of the forward and backward waves are orthogonal. As a result, the birefringence’s Δn of the system for forward and backward waves have opposite signs, which cause the opposite CD slopes for the forward and backward waves. So the theoretical value of CD in this design is zero.
3. Measurement results
Figure 5 is a photograph of the birefringent Michelson interferometer based interleaver with two-stage interferometer (without walk-off crystals and collimators). During manufacture all of the optical parts (except of the tuning plates) are first optically bonded together as an optical block. The blocking of the optical parts makes the optical alignment very easy: one only needs to align walk-off crystals and collimators with this optical block. In addition, during measurement only the angular position of the tuning plates to be adjusted for correct free-spectral-range (FSR) of the device. The tuning plate is made of quartz or special glass [20] that has the function of temperature compensation. The device is sealed in a package for thermal stability.
Fig. 5 A photograph of the birefringent Michelson interferometer based interleaver with two-stage interferometer (without walk-off crystals and collimators). The wave plates are too thin to see in the picture. The dimension is 10x9x5 mm3 (LxWxH).
The Agilent 86038B Photonic Dispersion and Loss Analyzer was used to measure Group Delay (GD), Chromatic Dispersion (CD) and Polarization Mode Dispersion (PMD), Insertion Loss (IL), and Polarization Dependent Loss (PDL). The measured results are shown in Fig. 6 and listed in Table 1 with designed values. The insertion loss (IL) and the ripple of IL are 1.3dB and 0.3dB, respectively. The 0.5 dB passband and 25 dB stopband both are 27 GHz, which almost equal the designed value of 28 GHz. The channel isolation is higher than 35dB which is very close to the designed value of 40 dB. The measured CD is less than ± 5 ps/nm, which is not the designed value of zero because the tolerances of the manufacturing still cause a small amount of CD. However, a CD of ± 5 ps/nm for a 50GHz interleaver is still much smaller than that of other interleaver technique. In this design we use some wave-plates which are wavelength-dependent elements. However, the wave-plates used in this design are very thin (less than 0.1mm) and their performances are not sensitive to wavelength in C-band. Figure 6 shows that all parameters (including IL; ripple of IL; bandwidth; CD; isolation etc) are very uniform in whole C-band.
Fig. 6 The measured insertion loss and CD of the birefringent Michelson interferometer based interleaver (two-stage interferometer). a- short edge; b-middle and c- long edge of C-band.
Table 1. Specifications of Novel Interleaver with 50 GHz Spacing
Figure 7 shows statistical distributions of the measured values of some parameters (IL; ripple of IL and 3db bandwidth). In this statistics 38 samples were randomly selected from the pilot line. The most of the measured values meet the designing specifications. The statistical result shows that this design can be applied to mass production.
Fig. 7 Statistical distributions of the measured values of some parameters for 38 samples: a—insertion loss(IL); b—ripple of IL; c—3db bandwidth.
4. Conclusion
We have developed a novel optical interleaver and fabricated a 50GHz interleaver. The interleaver consists of polarizing beam splitters and waveplates. The designed interleaver with two-stage of interferometer in a 50 GHz channel spacing application exhibits a 0.5 dB passband and a 25 dB stopband both at 27 GHz; and a channel isolation higher than 35 dB. Importantly, our interleaver features an ultra-low chromatic dispersion at < ± 5 ps/nm. Another advantage of this design is that all of the optical components can be optically bonded together as an optical block, thereby resulting in numerous benefits including: robust structure; small size; low insertion loss; easy optical alignment and low labor cost. For a three-stage design one only needs to add a PBS, a QWM and a HWP to the two-stage structure, and the 0.5 dB passband would increase from 28 GHz to 38 GHz.
Acknowledgment
The authors thank Mr. John (Jiwu) Ling and Mr. Jimmy (Zhimin) Liu for many stimulating discussions.
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