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Abstract
Narrow-band tunable optical filters (TOF) based on three cascaded Fabry-Perot (FP) cavities are demonstrated. The FP cavities are fabricated with Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT), which is a transparent electro-optic ceramic. With a reflective design, the optical signal passes the three FP cavities twice. Thus we can design the FP cavities with a relatively low reflectivity, which enables it more tolerant to the loss in the cavities. Two types of TOF samples are fabricated. The specifications of the transmissive type TOF with dual-ports are tuning range >48nm, FWHM (full width half maximum) <0.06nm, insertion loss <3.87dB, crosstalk <−39dB, which meet the requirements by optical performance monitor (OPM) applications. The specifications of the reflective type TOF with single-port are tuning range >48nm, FWHM (full width half maximum) <0.1nm, insertion loss <2.82dB, crosstalk <−30dB, which can be employed in tunable fiber lasers for wavelength selection and tuning.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
In the dense wavelength division multiplexing (DWDM) optical fiber links, optical performance monitor (OPM) is required to assure the optical signal-to-noise ratio (OSNR), as shown in Fig. 1. The core component in an OPM module is a tunable optical filter (TOF), which is required to have characteristics of low loss, low crosstalk, narrow bandwidth and wide tuning range, as shown in Fig. 2. For the 50GHz channel-spacing DWDM system, the FWHM (full width half maximum) of the TOF is required to be <0.15nm. Meanwhile, the tuning range needs to cover the C+ band of 48nm (1524-1572nm). Thus, the finesse of the TOF should be >320. As the DWDM technologies sinking from long-haul network to metro-network in real deployment, the demand for narrow-band TOF is urgent and the requirement for cost down is even pressing.
There are variable approaches for a TOF, such as liquid crystal (LC) Fabry-Perot (FP) cavities [3–4], MEMS (Micro-Electro-Mechanical System) FP cavities [5–6], silicon photonics [7–9] and acousto-optics [10]. The TOFs based on silicon photonics are compact and more potential for cost down and the acousto-optic TOF is characterized by broad tuning range. However, they both suffer from high loss. The LC TOFs reported in [3–4] and the MEMS TOF reported in [6] have a FWHM of 0.38nm, 0.39nm and 0.38nm, respectively. They are designed for applications such as optical channel monitor or selection, while not for OPM. The MEMS TOF demonstrated in [5] has a FWHM of 6-10nm and loss of 11dB. Cris Bolle reported an electromagnetical TOF with fast response and low loss in [11]. However, the tuning range is only 700GHz (∼5.6nm). It is specially designed for TWDM-PON (tunable wavelength division multiplexing passive optical network) application, where the tuning range is required to cover four DWDM channels.
Regarding OPM application, the main approach for a narrow-band TOF in real deployment is based on free-space optics (FSO) [12]. The FSO-type narrow-band TOF consists of a diffraction grating for wavelength dispersion and a MEMS mirror to select the passband wavelength. In order to realize a narrow bandwidth, two lens pairs are required in the FSO system. One for beam expansion before incidence on the grating and another for beam compression before incidence on the MEMS mirror. Thus, the FSO system is rather complicate and adds to fabrication difficulty and cost, which does not meet the cost down requirement of the metro-network. Thus, new solutions are expected.
The requirement for cost down pushes on detour around the FSO system. FP cavity is an ideal approach, while the FWHM and the tuning range are hard to be both optimized in a single FP cavity. Cascade of multiple FP cavities is expected to provide both narrow FWHM and wide tuning range, as reported in [13–15]. LC and electro-optic materials are employed in the FP cavities for [13] and [14–15], respectively. For the LC TOF reported in [13], the measured transmittance, FSR and FWHM are ∼65%, 400nm and 4nm, respectively. Thus, the finesse is about 100, which is far less than the requirement of 320 by OPM application. Adding to the reflectivity of the FP mirror can improve the finesse. However, the transmittance will also decrease. The electro-optic TOF reported in [14] is a dual-cavity device with FSR > 100nm, while the loss is rather high.
Comparing to LC, electro-optic materials are characterized by fast response. The most popular electro-optic material is lithium niobate (LiNbO3), which is widely used in high-speed modulators for optical communication. However, LiNbO3 works based on the primary electro-optic effect and thus the driving voltage is rather high for the TOF application. The transparent ceramics, such as Pb1-xLax(ZryTi1-y)1-x/4O3 (PLZT) and Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT), works on quadratic electro-optic effect, which tends to reduce the driving voltage. Comparing with PLZT, PMN-PT has higher electro-optic coefficient, faster response, lower field-induced and polarization dependent scattering loss, which enable it more suitable for TOF application [16–17].
In this paper, we demonstrate our work on TOFs based on cascaded FP cavities with PMN-PT material. Two types of TOF samples are fabricated. One is transmissive type with dual-ports, which can be employed in OPMs for 50GHz channel-spacing DWDM system. The other is reflective type with single-port, which can be employed in tunable fiber lasers for wavelength selection and tuning.
2. Concept and design
2.1 Basics for Fabry-Perot cavity
The frequency dependent transmission of a FP cavity can be expressed as Eq. (1) with finesse S as Eq. (2) [18].
For the OPM employed in 50GHz channel-spacing DWDM system, the TOF is required to be tuned over C+ band (ranging 1524-1572nm, i.e. FSR > 48nm) and the FWHM needs to be <0.15nm. Thus the cavity finesse is S = 320, which require the mirror reflectivity to be R = 99%. Such a high-finesse FP cavity will be too lossy.
What’s more, the FP cavity needs to be tuned by adjusting the refractive index (RI) of the cavity material, such as Eq. (3).
In order to cover the C+ band, the RI needs to be tuned by 3.1% (δf/fc = 4.8nm/1548nm = 3.1%), which is too much for the electro-optic materials. Thus, with loss and tunability considerations, a TOF for OPM application can’t be realized through a single FP cavity with electro-optic materials.
2.2 Cascaded Fabry-Perot cavities
A stack of FP filters with small FSR can reach the TOF specifications demanded by OPM application. In order to meet the requirements of narrow-band, wide tuning range, low loss and low crosstalk, the thickness of each cavity is limited to be <200µm and the mirror reflectivity should be 78%∼85%, according to simulation and former experiments. What’s more, the cavity thickness cannot be too thin because of fabrication difficulty. Thus we search for the optimized thickness of the three cavities within the range of 100∼200µm.
For the cascaded FP cavities, the suppression of crosstalk is realized by introducing differentiation between the interference orders of the cavities and thus the interference orders need to be elaborately designed. Moreover, the fabrication error on the cavity thickness will affect the interference order and thus results in uprising of crosstalk. Thus the fabrication error should be taken into account in the design of interference orders.
Given the thickness limitation above mentioned, the interference order m1 of the thickest FP cavity is fixed. Then the differentiation Δm2 = m1–m2 and Δm3 = m1–m3 of the other two cavities is simulated and the result is shown in Fig. 3. The white area in the figure that corresponds to the permissive value of Δm2 and Δm3 meet the requirement of crosstalk <−26dB. A broader continuous white area means more tolerance to error of interference orders and fabrication process.
Fig. 3. Simulation result of the interference order differentiation with restriction of crosstalk <−26dB
According to above simulation result, we cascade three FP cavities with the same finesse S = 14.05 (with mirror reflectivity R = 80%) and different interference orders m1 = 524, m2 = 470, m3 = 444 (i.e. FSR1 = 2.95nm, FSR2 = 3.29nm, FSR3 = 3.49nm). A mirror is placed after the cascade of cavities, and thus the optical beam passes each cavity twice. The simulated spectra of each single cavity and the cascaded cavities (by dual-pass) are shown in Fig. 4. The key specifications are FSR (i.e. tuning range) >48nm, FWHM∼0.08nm, crosstalk <−30dB. The finesse of the TOF spectrum is ∼600 (48nm/0.08nm), which is far more than the requirement of 320. However, the finesse of a single FP cavity is only 14.05. Thus the peak transmission is tolerant to the cavity loss.
2.3 Tuning characteristics analysis
For a single FP filter, the tuning characteristics can be obtained by differentiating 2nh = mc/fc, and it has the form:
According to Eq. (4), the tuning of resonant frequency fc can be realized by adjusting the refractive index n and the interference order m. Considering the limited tuning range of the electro-optic materials, the RI tuning is required to cover a single FSR. When the tuning range exceeds an FSR, a shift to the neighboring longitudinal mode is added to the RI tuning. Thus, combining the two tuning modes, the transmissive frequency can be expressed as follow.
where m0 is the initial longitudinal mode number of the FP filter, q is the mode shift, n0 is the initial RI of the cavity material, and h is the cavity length. PMN-PT is employed as the FP cavity material. When sintered into quasi-cubic crystal structure (trigonal phase), PMN-PT presents significant quadratic electro-optic effect. With electric field exerted, it is no longer isotropic and functions as a single-axis crystal. The RI along each axis is as follow. where the z-axis is along the direction of the applied electric field, no is the isotropic RI when no electric field is exerted, E is the intensity of electric field, S11 and S12 are the electro-optic coefficients along z-axis and its orthogonal direction, respectively. According to our experiment on PMN-PT, the electro-optic coefficients S11=−2.49×10−14(m/V)2 and S12=−2.65×10−14(m/V)2, and the isotropic RI is no =2.518.In our design, the direction of electric field is parallel to the optical beam, as shown in Fig. 5. Thus birefringence does not happen and the RI can be tuned according to Eq. (6).
3. Experimental details
3.1 Single Fabry-Perot cavity
The structure of a single FP cavity is shown in Fig. 6. The size of the PMN-PT plate is 5mm×5mm with thickness of 100∼200µm (three FP cavities have different thickness). Both sides of the PMN-PT plate are polished and then coated with indium tin oxide (ITO) film and partial reflection (PR) film. Note that the PR film covers the area of 3mm×5mm with part of ITO exposed for electrical connection. The FP cavities are fixed on a ceramic base. Each cavity is electrically connected through two copper pins.
3.2 TOF structure A
The first TOF is reflective type consisting an optical circulator, a fiber collimator, three cascaded FP cavities and a mirror, as shown in Fig. 7 and Fig. 8. Optical signal from the fiber collimator passes the FP cavities twice through reflection by the mirror. The input and output are separated by the optical circulator.
Fig. 7. Schematic diagram of the reflective type TOF with a single fiber collimator and an optical circulator
The measured spectral transmission of the reflective type TOF is shown in Fig. 9, the specific experimental results are shown in Table 1, and the zoomed in view on the spectrum is shown in Fig. 10 The specifications are tunable range >48nm, FWHM < 0.1nm, loss <2.82dB (excluding the loss of the optical circulator), crosstalk <−30dB.
3.3 TOF structure B
The second TOF is transmissive type consisting two fiber collimators, three cascaded FP cavities and a rectangular prism, as shown in Fig. 11 and Fig. 12. Optical signal from the fiber collimator passes the FP cavities twice through reflection by the prism.
The measured spectral transmission of the transmissive type TOF is shown in Fig. 13, the specific experimental results are shown in Table 2, and the zoomed in view on the spectrum is shown in Fig. 14. The transmissive type TOF has higher mirror reflectivity for FP cavities comparing to the reflective type TOF (86% vs 78%). The specifications are tunable range >48nm, FWHM < 0.06nm, loss <3.87dB, crosstalk <−39dB.
3.4 Discussion on the experimental results
The measured specifications of the two types of TOF are summarized in Table 3. The reflective type has lower loss, while the transmissive type has narrower FWHM and lower crosstalk. The two TOFs are both realized by dual-passes through three FP cavities, while the cavities are fabricated in two batches. The difference in TOF specifications is due to the unstable fabrication process of the FP cavities. The process for ITO film coated on PMN-PT is rather difficult and the ITO film becomes even slimsy after the PR film is coated thereon, which leads to defective ohmic contact and relatively high bias voltage. The defects and nonuniformity on the ITO and PR films introduce light absorption and scattering and decrease the reflectivity.
Table 1. Experimental results of the reflective type TOF
Table 2. Experimental results of the transmissive type TOF
Table 3. Measured specifications of the TOFs
Actually, we measured loss as low as 0.2dB at some point of a single FP cavity. However, the incident point can’t be selected for the FP cavities assembled in the TOFs. The quality and uniformity of the ITO and PR films are expected to be improved after more experiments on the coating process. Thus the loss of the TOF is expected to be <2dB.
4. Conclusion
An approach for narrow-band TOF is presented. The devices are realized by cascaded FP cavities and the cavities are fabricated with material PMN-PT. A transmissive type TOF is fabricated for OPM application and another reflective type is designed for employment in a tunable fiber laser. Both the two samples are characterized by wide tuning range, narrow bandwidth, low loss and low crosstalk. The specifications are expected to be further improved after optimization of the fabrication process for the FP cavities. Comparing to the current TOF based on free-space optics, the devices we presented are potential for cost down.
Disclosures
The authors declare no conflicts of interest.
Data availability
No data were generated or analyzed in the presented research.
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