A two-in-one Faraday rotator mirror was presented, which functions as two independent Faraday rotation mirrors with a single device. With the introduction of a reflection lens as substitution of the mirror in traditional structure, this device is characterized by exemption of active optical alignment for the designers and manufacturers of Faraday rotator mirrors. A sample was fabricated by passive mechanical assembly. The insertion loss was measured as 0.46dB/0.50dB for the two independent ports, respectively.
© 2014 Optical Society of America
Faraday rotator mirrors (FRM) are widely used as passive optical devices for the purpose of stabilizing the operation of optical fiber interferometer , optical fiber sensing system [2–4], optical fiber amplifier system , optical fiber laser source or the like [6,7]. In some applications, two FRMs are required for a single system [1,2].
Inside a passive optical device, the individual components need to be well aligned to reduce the power loss. The alignment process usually needs a laser source and a photon detector to monitor the power loss in the device. The components are aligned with high-precision mechanical stages under monitoring. Such alignment process is usually called active optical alignment. In comparison, the alignment process that doesn’t need the laser source and photon detector for monitoring is usually called passive mechanical assembly, which requires larger alignment tolerance.
The present approaches for a FRM device usually comprise a single mode fiber, a collimating lens, a Faraday rotator, a magnet ring and a mirror. The mirror needs to be well aligned perpendicular to the direction of the collimated beam, which requires active optical alignment and is ineffective.
This article reports a two-in-one FRM device, which functions as two independent FRMs with a single device and thus simplifies applications in the systems such as [1,2]. With a reflection lens as substitution of the mirror in traditional structure, the presented device is characterized by exemption of active optical alignment. That’s to say, it can be realized simply by passive mechanical assembly for the designers and manufacturers of FRM devices.
2.1 Operation principles
The structure of the two-in-one FRM is shown in Fig. 1, comprising a dual-fiber collimator, a 45° Faraday rotator, a reflection mirror and a magnet ring. The dual-fiber collimator comprises two single mode fibers, a glass capillary, a collimating lens and a glass tube for assembly. The collimating lens is a rod-shaped plano-convex lens with the planar endface slantly polished to improve return loss. The reflection lens is also a rod-shaped plano-convex lens, while the planar endface is coated with high-reflection (HR) coating.
The two fibers of the dual-fiber collimator function as the input/output ports of two independent FRMs. The input optical signals are first collimated by the collimating lens. The collimated beams pass through the Faraday rotator and are then reflected by the reflection lens. Based on special design of the parameters, the reflected beams backtrack to the dual-fiber collimator and then go out from the respective input fibers.
The structure and parameters of the dual-fiber collimator are shown in Fig. 2. In the top view, the two optical fibers are aligned in parallel and fixed by the glass capillary. The endface of the fibers locates at the front focal plane of the collimating lens and the two collimated beams symmetrically cross at the rear focal plane with an angle ϕ. In the side view, the two fibers and the respective beam traces coincide. The collimated beams cross the rear focal plane at a height r1 with an angle θ1, depending on parameters of the optical fiber and the collimating lens. Based on ray tracing and approximation, r1 and θ1 are obtained as Eqs. (1) and (2).
According to Eq. (1), r1 = 0 when Eq. (3) is satisfied, which means that both the two collimated beams pass through the cross point O between the optical axis and the rear focal plane of the collimating lens.
The parameters of the reflection lens are shown in Fig. 3. The refractive index, length and endface curvature radius of the lens are nr, Lr and Rr respectively. The front focal length is fr = Rr/(nr-1) and the length is lr = nrfr. Thus the HR coated endface locates just at the rear focal plane of the lens.
When inspecting at the front focal plane of the reflection lens, the ABCD transmission matrix between the output and input ray parameters is:
Equation (4) gives r2 = -r1 and θ2 = -θ1, which means that the reflected beam 2 by the reflection lens is parallel to the incident beam 1 and the height of the two beams at the front focal plane is symmetric about the lens axis, as shown in Fig. 3(a). Thus the reflected beam backtracks the incident beam when the height of the incident beam is r1 = 0, as shown in Fig. 3(b).
During assembly of the FRM device, the front focal plane of the reflection lens superimposes the rear focal plane of the collimating lens. When Eq. (3) is satisfied, the two collimated beams by the collimating lens backtrack themselves after reflected by the reflection lens. Thus the two-in-one FRM is exempt of optical alignment by mechanical stages with high precision.
2.2 Tolerance analysis
Passive mechanical assembly of an optical device doesn’t need the light source and photon detector. The power loss is minimized simply with high-precision mechanical assembly, which helps to improve the fabrication efficiency. In an optical device, a minor misalignment usually causes considerable power loss. Thus most optical devices are assembled by active alignment. Passive alignment is desired, while it needs larger alignment tolerance.
The two-in-one FRM is characterized by exemption of active optical alignment theoretically, i.e., the alignment tolerance is large enough to ensure low power loss by passive assembly. The main misalignment factors that may cause excess power loss in the device include: the axial misalignment δZ between the front focal plane of the reflection lens and the rear focal plane of the collimating lens, and the lateral misalignment δX between the axes of the two lenses, as shown in Fig. 4. The misalignments are introduced by mechanical errors during assembly.
For two Gaussian beams with lateral misalignment D, the coupling loss can be given as 
When inspecting at the front focal plane of the reflection lens, the misalignment δZ and δX result in lateral misalignment Di (i = 1, 2) between the incident and reflected beams, as shown in Eqs. (6) and (7), respectively.
According to Eqs. (5)–(7), the excess power loss resulting from δZ and δX is obtained as Fig. 5 and 6. The magnitude of misalignments corresponding to excess power loss of 0.1dB is δZ = 2.5mm or δX = 27μm. The axial alignment tolerance is large enough and can be easily guaranteed by mechanical assembly. The lateral alignment tolerance is relatively strict. It can be fulfilled by well control of the fabrication tolerance of the inner diameter of the magnet ring and the outer diameters of the collimating and reflection lenses.
3. Experimental results
A two-in-one FRM device was fabricated. The dual-fiber collimator is a commercially available one by Photop Technologies with SMF-28 optical fibers. The refractive index of fiber core is nf = 1.4682 for λ = 1.55μm. The slant angle of fiber endface is α = 8°. The material for the collimating lens is SF11-type glass by Schott. The refractive index is nc = 1.7447 for λ = 1.55μm. The curvature radius of the rear endface is R = 1.419mm and thus the focal length is fc = 1.905mm. The slant angle of the front endface was customized as φ = 5°. The outer diameter of the collimating lens is ODc = 1.8 + 0/0.005mm. Thus the radius of the collimated beams is ω = λfc/πω0 = 181μm. The insertion loss provided by the supplier is <0.20dB for both ports.
The reflection lens was customized by Newsandgy Optronics. The material is also SF11 glass by Schott. The curvature radius of the front endface is also R = 1.419 and thus the focal length is fr = 1.905mm. The length is Lr = nrfr = 3.32mm and outer diameter is ODr = 1.8 + 0/0.005mm. The front endface is AR (anti-reflection) coated and the rear endface is HR coated.
The Faraday rotator is GSF-type by Granopt. The specifications show that the insertion loss is <0.1dB for λ = 1.55μm. The saturation field is 200Oe, i.e., the rotation angle is fixed as 45° when the magnet field exerted is >200Oe. The magnet ring was provided by Jiaozuo Micro Magnet Technology. The outer diameter is ODm = 2.8mm and inner diameter is IDm = 1.8 + 0.02/-0mm. The height of the magnet ring is 4.0mm. Specifications by the supplier show that the magnetic field is >600Oe inside the magnet ring.
The Faraday rotator was pre-assembled in the magnet ring as a Faraday unit. Then the dual-fiber collimator, the reflection lens and the Faraday unit were assembled by passive mechanical alignment. The final assembly is shown as Fig. 7.
The test system for the FRM device is shown in Fig. 8. The two ports are tested one by one. An optical circulator is employed to separate the optical signals input to and output from the FRM device. An in-line fiber polarization controller is employed to test the polarization dependence. A polarizer is employed, which is coupled with two fiber collimators on both sides and thus is pigtailed with optical fibers. It can be connected into the system by fiber splicing.
When measuring the power loss, the polarizer is not connected into the system. Point A is connected to the power meter directly for power reference. Then the FRM device is connected with port 2 of the optical circulator. The power loss is measured under different polarization by adjusting the polarization controller. The maximum power loss was measured as 0.46dB/0.50dB (the insertion loss between port 2 and 3 of the optical circulator was deducted) for the two ports, respectively. The polarization dependent loss (PDL) is 0.04dB/0.06dB.
The sources of power loss include: loss in the dual-fiber collimator ~0.2dB, absorption by the Faraday rotator ~0.1dB, loss in the reflection lens ~0.1dB, and loss due to misalignment ~0.1dB.
The polarization state of the reflected light from the FRM device should be rotated by 90°. In order to verify that the output polarization state is as expected, another test was performed. The polarizer was connected into the system between points A and B. Point B is connected to the power meter directly for power reference, which was maximized by adjusting the polarization controller. Then the FRM device is connected to point B. The measured isolation is 28.7dB/29.4dB (the insertion loss between port 2 and 3 of the optical circulator and the insertion loss of the polarizer were deducted) for the two ports, respectively.
The operation principle of a two-in-one FRM device was presented, which is characterized by exemption of active optical alignment. The alignment tolerance was analyzed, which theoretically supports that the device can be assembled by passive mechanical alignment. A device sample was fabricated by passive assembly and the power loss was measured as 0.46dB/0.50dB for the two ports, respectively. The two-in-one structure simplifies the applications in the optical fiber sensing systems and the exemption of active optical alignment helps to improve the fabrication efficiency.
This work has been co-supported by National Natural Science Foundation of China (No. 61107062) and the Fundamental Research Funds for the Central Universities of China.
References and links
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