We propose and demonstrate a novel Sagnac interferometer based flat-top birefringent optical interleaver employing a ring-cavity as a phase-shift element. The Sagnac interferometer with birefringent crystals provides the optical path difference for interference between the two orthogonal polarization components and the ring-cavity provides the phase shifts needed to achieve a flat-top spectral passband at the output. Fresnel reflections at the prism-air interface of the ring cavity are employed to obtain the desired phase shifts so that highly accurate thin-film coatings are not needed. The Sagnac interferometer based interleaver in a 25-GHz channel spacing (0.2 nm) application exhibits a 0.5-dB passband larger than 0.145 nm, a 25-dB stop band greater than 0.145 nm, and a channel isolation higher than 36 dB over the entire C-band. The superior performance is accompanied with a group velocity dispersion and ripples that can be compensated by using dispersion compensators.
©2006 Optical Society of America
An important and convenient approach to further increase the channel count is to introduce interleavers in conjunction with conventional Mux/Demux filters. Optical interleavers are enabling components in Dense Wavelength Division Multiplexing (DWDM) optical communications when the channel spacing is in the range of 50 GHz (0.4 nm) or less. An interleaver is very useful because it can relax the specification requirements for demultiplexers in DWDM systems. An interleaver is capable of separating a set of channels into two sets at twice the channel spacing. In reverse operations, the interleavers are employed in DWDM systems for packing as many channels as possible into fiber-optic networks . The optical interleaver has been proven to be an effective way in increasing the capacity by doubling or quadrupling the number of channels, for system upgrade to terabits capacities .
By virtue of the narrow channel spacing (e.g., 25 GHz), all interleavers are made of optical interferometers. Phase-dispersion elements serving as mirrors are employed in the interferometers to provide flat-top spectral passbands. These phase-dispersion elements are made of optical resonators such as Gires-Tournois etalons (GTEs) or ring cavities [3–10]. To provide a uniform performance over the entire C-band, the partial reflecting mirrors of the optical resonator must maintain constant reflectivities over the entire C-band. Such requirements are difficult to achieve by using conventional thin-film coating technology. This problem was solved recently by using a ring cavity which employs a prism as the partial reflecting mirror . The prism interface of the ring cavity is employed as the partial reflecting mirror whose reflectivities are determined by the Fresnel reflections (for s- and p-polarization states). These reflectivities are virtually constant over the entire C-band. Such a ring cavity provides two phase shifts (one for each polarization component) needed to obtain flat-top passbands. By choosing the appropriate incident angle (inside the prism) near the Brewster angle, we can obtain the optimum interface reflectivities needed for flat-top passbands and extremely low crosstalk. However, four additional crystals were needed for separating and recombining the polarization components. These additional crystals must be optically aligned to avoid insertion loss. This leads to insertion loss and performance degradation due to misalignments in the whole assembly.
In this work, we proposed and demonstrated a novel Sagnac interferometer based birefringent optical interleaver utilizing a ring-cavity architecture to ensure better flat-top transmission passbands, as well as superior channel isolation. The Sagnac interferometer based optical interleaver with birefringent crystals is used to provide the optical path difference for interference between polarization components. The loop design requires only a single polarization beam splitter (PBS) for separating and recombining the polarization components. This new architecture requires less number of components and thus leads to a lower insertion loss. We have experimentally demonstrated the interleaver in a 25-GHz channel spacing (0.2 nm) application with a wider 0.5 dB passband (flat-top), a greater 25 dB stopband, and a higher channel isolation which were suitable for capacity upgrade in DWDM applications. The results of the Sagnac interferometer with novel ring-cavity structure can simultaneously produce a 0.5-dB passband and a 25-dB stopband wider than those of other interleavers employing ring cavity.
2. Optimum design of ring-cavity based flat-top birefringent interleaver
Virtually all interleavers are made of optical interferometers. The interference creates an optical output which is a periodic function of frequency. For birefringent interleavers, the free spectral range is determined by the length and index difference of the birefringent crystal. Figure 1 shows a schematic drawing of the new architecture of the birefringent optical interleaver. The interleaver was made of a novel Sagnac interferometer with ring cavity architecture. In addition to the polarizing beam splitter (PBS), the birefringent interleaver consists of a ring cavity, two birefringent crystals (YVO4), two half-wave plates, and two mirrors. The Sagnac interferometer was formed by a single PBS, two birefringent crystals (YVO4), two half-wave plates, two mirrors (M3 and M4) and a prism. The ring cavity consists of two mirrors (M1 and M2) and a prism.
A beam of unpolarized light is separated into two polarized beams by the PBS. These two polarized beams, designated as s- and p-components, are propagating in the opposite directions within the loop. The s-component is polarized perpendicular to the plane of propagation, and the p-component is polarized parallel to the plane of propagation. The s-component propagates along the loop in the clockwise direction and the p-component propagates along the loop in the counterclockwise. Two half-wave plates are employed to rotate the polarization states of these two beams by 45 degrees. After transmitting through the half-wave plates, the beams are directed toward two YVO4 birefringent crystals with the appropriate retardance for interference. The c-axis of the YVO4 birefringent crystals is oriented parallel to the plane of propagation. As a result, the beams inside the birefringent crystals consist of both the ordinary wave and the extraordinary wave with an equal amplitude. As the beams propagate inside the birefringent crystals, a phase retardation exists between these two waves at the end of the crystals. These beams, consisting of both ordinary wave and extraordinary wave, are then directed toward the ring cavity. In our interleaver as shown in Fig. 1, the ordinary wave corresponds to s-wave while the extraordinary wave corresponds to p-wave. The prism interface of the ring cavity exhibits different Fresnel reflectivities for these two polarization components (s- and p-) of the beam. As a result of the different reflectivities, these two polarization components experience different phase shifts upon transmitting (or reflecting) through the ring cavity. After the ring cavity, these two components of the beam experience further phase retardation from the second birefringent crystal before they are mixed and recombined by the half-wave plate and the PBS. The Sagnac interferometer with birefringent crystals provides the optical path difference for interference between polarization components. The ring-cavity provides the phase shifts needed to achieve a flat-top spectral passband at the output.
2.1. Optical spectrum of the birefringent interleaver
In our birefringent interleaver, the ring cavity, acting as a GTE, was made of mirror-1 (M 1), mirror-2 (M 2) and the prism-air interface which was aligned perpendicularly to the optical beams. The prism-air interface serves as the partial reflecting mirror of the ring cavity. The prism provides the appropriate angle of incidence so that the desired Fresnel reflectivities, R s and R p, could be obtained. A simple analysis of the transmission in the birefringent optical interleaver as shown in Fig. 1 yields the following intensity at one of the output ports [9, 11]
where I 0 is the intensity of the incident beam (unpolarized), L 1 and L 2 are the length of the two birefringent crystals respectively, ϕ e and ϕ o are the phase shifts of the beam upon reflection from the ring cavity, n e and n o are the refractive indices of the birefringent crystals. We note the ordinary wave corresponds to the s-wave, while the extraordinary wave corresponds to the p-wave in our interleaver. The phase shifts can be written as [9, 11]
where ϕ=2πν L R/c is the round-trip phase shift of light inside the ring cavity, L R is the roundtrip beam path in the optical cavity and R o=R s, R e=R p. By examining eq. (1), we note that the intensity is a sinusoidal function of frequency ν if the phases (ϕe and ϕo) are constants. The period of the sinusoidal function is
According to Eq. (2), the phase shifts are also periodic function of frequency ν with a period of Δν R=c/L R. As a result, the output transmission, in general, is no longer sinusoidal. To ensure a periodic transmission function, these two periods must be identical. Thus, the roundtrip beam path in the optical cavity must be related to the total crystal length by
2.2. Ring cavity architecture
A proper variation of the phase shifts relative to the frequency can transform the sinusoidal output transmission into a near square-wave transmission output [4–10]. Furthermore, a series of flat-top transmission passbands can be obtained when ring cavity provided a proper variation of the phase shifts. This requires a proper choice of the reflectivities of the partial reflection of the prism-air interface. Figure 2 is a schematic drawing of the ring cavity. In Fig. 2, L R is the round-trip optical path length inside the ring cavity, M1 and M2 are highly reflective mirrors, θ 1 and θ 2 are the incident and transmitted angles. The ring cavity, acting as a Gires-Tounois etalon (GTE), is formed by the M1, M2 and the prism-air interface; the mirrors are high reflectors and are aligned perpendicularly with the light beams. The dispersion of the ring cavity exhibits a periodic dependence on the frequency of light. The important characteristic of the prism shape is that it must have two surfaces which allow the input beam and output beam to exit perpendicularly to those two surfaces, and must have a third surface which acts as a front mirror to reflect the beam at an angle θ 1 with respect to a line perpendicular to the third surface. The input beam incidents perpendicularly to the first surface, and reflects from the second surface, and then transmits through the third surface perpendicularly. At the air-prism interface, the two modes (extraordinary and ordinary modes) meet different reflectivity and can be expressed as
where R e is the reflectivity of the air-prism interface for the e-component (extraordinary beam), R o is the reflectivity of the air-prism interface for the o-component (ordinary beam), θ 1 and θ 2 are the incident and transmitted angles, n1 and n2 are the reflective indices of prism and air respectively.
2.3. The group delay and chromatic dispersion of the birefringent interleaver
The superior performance of the intrleaver is accompanied with a chromatic dispersion which includes a group velocity dispersion and ripples. As the data rate of fiber optical communications increases to 10 Gb/s or higher, such a chromatic dispersion (CD) must be eliminated. This is often achieved via the use of a dispersion compensator. The chromatic dispersion of interleavers normally has two origins. One is the material dispersion; the other is the structural dispersion originated from the ring cavity. In most interleavers, chromatic dispersion is dominated by the structural dispersion instead of the material dispersion since the length of the material used to construct the interleaver is relatively short .
The group delay τi(ω) is defined by -dφι(ω)/dω , where i=e or o. The output group delay from the birefringent interferometer can be viewed as the average group delay from two modes, τ(ω)=[τe(ω)+τo(ω)]/2, and can be derived from the Eq. (2) as
where ω=2πc/λ is the optical angular frequency, T=L R/c is the round-trip time in the ring cavity, R e and R o are the reflectivity of the air-prism interface for the e-component (extraordinary beam) and o-component (ordinary beam), and λ=c/υ. The group velocity dispersion (GVD) is given by D(λ)=dτ/dλ [ps/nm]  and can be derived from the Eq. (6) as
3. Simulation results
3.1. Flat-top interleaver
Based on Eq. (1)–Eq. (5), the simulation results of the transmission spectrum for the incident angles (θ 1)=31, 31.65, and 32 degrees in the 25-GHz channel spacing application are showed in Fig. 3. Figure 3(a) shows that the flat-top ripple of the incident angles (θ 1)=31, 31.65, and 32 degrees are 5.5×10-3 dB, 0.2×10-3 dB and 2.5×10-3 dB, respectively. Figure 3(b) shows that the simulation results of the 0.5-dB passband, 25-dB stopband and channel isolation for the incident angles (θ 1)=31, 31.65, and 32 degrees in the 25-GHz channel spacing application. The 0.5-dB passband were found to be 0.18, 0.182, and 0.186 nm for the incident angles (θ 1)=31, 31.65, and 32 degrees, respectively. Figure 3(b) shows the 25-dB stopbands of the incident angles (θ 1)=31, 31.65, and 32 degrees are 0.154, 0.155, and 0.157 nm, respectively. Figure 3(b) also shows the channel spacing for the incident angles (θ 1)=31, 31.65, and 32 degrees in the 25-GHz channel spacing application are 29, 45, and 32 dB, respectively. In this study, the optimum incident angle in the prism is 31.65 degree which leads to the optimum mirror reflectivities of R o=41.99% and R e=3.3%. The flat-top ripple, 0.5-dB passband, 25-dB stopband, and channel isolation are 0.2×10-3 dB, 0.182 nm, 0.155 nm, and 45 dB, respectively, in a 25-GHz channel spacing (0.2 nm) interleaver application.
3.2. Chromatic dispersion
Group velocity dispersion is an intrinsic property of a cavity or resonator. At resonance, the group delay reaches its maximum. Both the group delay and the group velocity dispersion are periodic function of the frequency. sing Eq (5)–Eq. (7), the simulations of the group delay and chromatic dispersion for birefringent interleaver in a 25-GHz channel spacing (0.2 nm) are shown in Fig. 4. Figure 4(a) indicates the group delay ripple across the 25-GHz passband is 16.73~122.51 ps. Figure 4(b) shows that the chromatic dispersion ripple across the 25-GHz passband is about +/- 4438 ps/nm. For application in optical networks, such group delays must be compensated by using dispersion compensators.
4. Measurement results and discussion
In our experimental investigation, we employed optical fiber collimators at both of input and output ports. For a 25-GHz channel spacing (0.2 nm) interleaver, the total length of the two crystals was approximately 30 mm. An air cavity was employed for athermal purposes. A broadband light source and an optical spectrum analyzer were used for the interference measurements. The measured spectral transmission characteristics of the 25-GHz interleaver at one of the output ports are shown in Fig. 5. Figures 5(a) and 5(b) show the channel isolation, 0.5-dB passband, and 25-dB stopband for the 2 nm band and the 10 nm band, respectively, in a 25 GHz channel spacing. We note that the 0.5-dB passband and the 25-dB stopband of the interleaver with a 25-GHz channel spacing (0.2 nm) are 0.145 nm (18.1 GHz), and the insertion loss is 1.5 dB. Figure 5 also shows the channel isolation is better than 36 dB between the adjacent channels.
We attribute the difference between the simulation and experimental results to a slight misalignment along the ring cavity and the optical loop. Compared to our previous work of the birefringent interleaver with a ring cavity , this work has more than 2.5% increase in the bandwidth utilization as well as 0.7dB improvement in the insertion loss. These improvements in the performance are results of a less number of optical components which significantly eases the optical alignment. In this study, the optical beams are directed into a loop (like a Sagnac interferometer) and a ring cavity. The loop design requires only a single polarizing beam splitter (PBS) for separating and recombining the polarization components. In our previous work , four additional crystals were needed for separating and recombining the polarization components. These additional crystals must be optically aligned to avoid insertion loss.
Although a single birefringent of the proper length is adequate, the use of two separate crystals has the advantage of compact packaging as well as athermal engineering design. Two different birefringent crystals with opposite thermal properties can be employed to cancel the thermal-optical effects. Furthermore, mirrors can be eliminated by using birefringent crystals cut with prism interfaces to provide total internal reflections. The net decrease in the number of optical components can further improve the performance, including lowering the insertion loss.
In summary, we have proposed and demonstrated a flat-top 25-GHz optical interleavers based on a new architecture involving a Sagnac interferometer and a ring cavity architecture. The spectral transmission characteristics of the interleaver with a 25-GHz channel spacing (0.2 nm) exhibited a 0.5-dB passband greater than 18.1 GHz (72.5% of the channel spacing), a 25-dB stopband greater than 18.1 GHz (72.5% of the channel spacing), and a channel isolation better than 36 dB. The new Sagnac interferometer architecture with birefringent crystals requires less number of optical components and hence results in more than 0.7 dB and 2.5% improvements in the insertion loss and the band utilization, respectively, when compared to our previous birefringent interleaver with a ring cavity .
The uniform performance over the entire C-band is a result of the unique ring cavity architecture which utilizes Fresnel reflections of the prism-air interface to provide the partial reflections (for s- and p-polarization components). Unlike dielectric mirrors with thin-film coatings, the reflectivities of Fresnel reflection are insensitive to wavelength variations in the transmission band. The uniform reflectivities are essential to ensure the same performance over the entire C-band. In this work, we have demonstrated a high-performance birefringent optical interleaver using a single ring cavity and a Sagnac interferometer architectures in a compact optical path design. Furthermore, we have experimentally demonstrated a Sagnac interferometer based flat-top birefringent optical interleaver employing ring-cavity architecture as a phase-shift elements which could simultaneously produce a 0.5-dB passband and a 25-dB stopband wider than those of other interleavers employing ring cavity. The superior performance of the interleaver is accompanied with group velocity dispersions and ripples. Such dispersions and ripples can be compensated by using dispersion compensation elements.
The authors would like to thank Ming-Hung Chen for technical discussion, device setup, and sample preparation. This work was partially supported by the MOE program of Aim for the Top University Plan, and the National Science Council of the Republic of China under contracts NSC 94-2215-E-110-011.
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