1. Introduction

Optical rotary joints (ORJs) can be used for transmission of both digital and analogue signals through relatively rotating interfaces (RRIs). It has been used for transmission of digital signals in phase-arrayed antennas, fiber-guided vehicles, fiber-guided scanning cameras, and many other military or commercial applications [1–5]. It can also be used for transmission of analogue signals transparently through RRIs in optical sensing applications. Compared to its counterpart, the electrical slip ring (ESR), ORJ is immune to electo-magnetic interferences. Currently, its main disadvantage against ESRs is the high manufacturing and assembly cost, which is caused by the tight mechanical tolerance requirements. The cost of a two-channel ORJ can be as high as $4,000 for multimode optical data transmission and $10,000 for singlemode transmission [3].

To solve this problem, an ORJ was previously designed using Graded-index (GRIN) rod lenses and bulk photodetectors to relax the requirements on mechanical tolerance [4]. This ORJ structure is suitable for single-channel applications, where the data transmission rate is around 1 Gbit/s. But it cannot meet the requirements in certain applications where more than one data channel or a bandwidth of several gigabits/s is required. Several techniques have been adopted to design multi-channel ORJ, including diffractive optics, refractive optics and integrated optics [5,6]. Although these techniques can meet the bandwidth and data-channel requirements, these ORJ structures inevitably lead to high costs.

A broadband optical rotary joint (BORJ) was designed using C-lenses, which were being adopted more and more widely for collimating of light beams in optical communications, optical data transmissions, optical sensing, and optical inspection applications [7–11]. The overall working principle and structure of C-lenses is similar to that of GRIN lenses, with the main difference lying in the shape of their rear ends [9]. The rear ends of C-lenses are spherical, while those of GRIN lenses are planar. When the working distance is longer than 50 mm, the insertion loss and mechanical tolerance of C-lenses are better than those of GRIN lenses. Through the optimization of its mechanical design, an insertion loss of less than 2 dB at both the 1300 nm and 1550 nm wavelength windows was achieved in this BORJ with a mechanical tolerance of 10 µm. Wavelength division multiplexing (WDM) technique was adopted to increase the data transmission channels. The effective bandwidth of this BORJ is more than 170 nm, hundreds of wavelength channels can be accommodated for data transmission through relatively rotating interfaces. The total data transmission rate through this BORJ can reach hundreds of Gbit/s. By using Dove prism, both space division multiplexing (SDM) and WDM techniques can be implemented simultaneously in the design of BORJ with C-lens. This structure of BORJ has a low cost. And it can be adopted for optical data transmission and optical sensing.

2. Structure and theory of the broadband optical rotary joint

The working principle of the BORJ is shown in Fig. 1. It consists of two parts: the rotator and the stator. The rotator is mainly composed of a C-lens, a lens holder, two ball bearings, and auxiliary fixing and bushing mechanical parts. The stator mainly consists of a C-lens, a lens holder, and corresponding bushing and fixing mechanical parts. The rotator is connected to the stator via two ball bearings, which have a mechanical vibration of less than 10 µm in rotating. The optical beam injected from one end of the BORJ is collimated and expanded by one C-lens. Then it transmitted to the other C-lens, where it is focused and coupled into the singlemode optical fiber (SM fiber). To ensure a low insertion loss, say less than 3 dB, the mechanical parts of both the rotator and the stator should be mounted co-axially.

Similar to optical rotary joints implemented via GRIN lens, there are three sources for the insertion loss of this BORJ: lateral misalignment, axial separation, and angular tilting [4, 12–14], see Fig. 2.

 

Fig. 1. Working principle of the broadband optical rotary joint.

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To achieve an insertion loss under 3 dB with GRIN lens, the lateral misalignment, axial separation and angular tilting should be kept under 100 µm, 50 mm, and 0.1°, respectively [4]. Compared to GRIN lens that has been widely adopted in optical communications and optical sensing [15–19], C-lens has better optical properties. The refractive index of the core of C-lenses is dependent on the central working wavelength. It can be calculated as follows

where n ₀ is the material refractive index, and λ₀ is the central wavelength in micrometer. The focusing constant is

where is p is the pitch of the lens, and c is a constant depending on the working distance and central wavelength. The value of c decreases with the increase of the working distance. For a working distance of less than 85 mm and central wavelength of 1550 nm, as the case in this design, the value of c should be around 0.237. The focal length is

And the length of the lens is

For Gaussian beam in single-mode fibers, its field radius is approximately 5.25 µm [12–14]. After passing through the C-lens, the field radius of the collimated beam is about 200 µm. The insertion loss caused by a couple of C-lenses is calculated to be less than 0.5 dB for an angular tilting under 0.15° according to the theory of wave optics [12,13]. Similarly, the tolerance for lateral misalignment is calculated to be about 250 µm to achieve an insertion less under 0.5 dB. Since the working distance of C-lenses is more than 100 mm for an insertion loss of 0.5 dB, the axial separation induced insertion loss in the BORJ is neglectable compared to those caused by angular tilting and lateral misalignment.

 

Fig. 2. Three sources for the insertion loss of the C-lenses.

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To test the feasibility of the ORJ structure shown in Fig. 1, a BORJ was designed and fabricated. The maximum insertion loss of this BORJ is less than 1.8 dB, and the variation of insertion loss in rotating is less than 0.5 dB at 1310 nm. The insertion loss of the BORJ at different rotation angle is shown in Fig. 3. The working distance of the C-lenses is less than 20 mm and the lateral misalignment of the C-lenses is less than 20µm in this design. So the insertion loss is mainly caused by the angular tilting of the C-lenses. The variation of the insertion loss is mainly caused by mechanical vibration induced changing of angular tilting in rotation. In the testing of the insertion loss, two singlemode FC-type fiber connectors were used to connect the BORJ to the optical multimeter and the optical power, respectively. So the maximum insertion loss of the BORJ without fiber connectors is about 1.6 dB at 1310 nm.

 

Fig. 3. Relationship between the insertion loss of the BORJ and its rotation angle.

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3. Wavelength division multiplexed optical rotary joint

To expand the channel number of the BORJ, wavelength division multiplexing (WDM) technique was adopted to increase the number of data channels passing through relatively rotating interfaces. The working principle of the WDM-based multichannel ORJ is shown in Fig. 4. The input optical signals with carrier wavelengths λ₁, λ₂, …, λ_n are first sent to the multiplexing module, then sent to an optical isolator where the input signals are isolated from the output signals from the BORJ to reduce the noise of this rotational data transmission system. At the other end of this BORJ, the output signals transmit through an optical coupler to the demultiplexing module where these signals are demultiplexed and sent to the output channels. With this wavelength division multiplexed ORJ, n optical channels can be transmitted bidirectionally through relatively rotating interfaces.

 

Fig. 4. Principle of the wavelength division multiplexed ORJ.

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The total effective bandwidth of this wavelength division multiplexed ORJ is proportional to the number of input/output optical channels n and the effective bandwidth of each channel B_ch. The channel number n should meet the following relationship

where B_gap is the gap of bandwidth between two adjacent optical channels to reduce noise and ensure safe data transmission, and B_total is the total bandwidth of the BORJ that can be used for data transmission. A tunable laser (Agilent 81640) was used to evaluate the chromatic dispersion of this BORJ. Its wavelength can be tuned from 1510 nm to 1640 nm. The optical signal from the laser is coupled into the BORJ with a FC-type fiber coupler. Then, the optical power of the output signal from this BORJ is measured with an optical multimeter (HP 8153A). The test results are depicted in Fig. 5. In this experiment, the working distance of the C-lenses is about 10 mm, and the lateral misalignment of the C-lenses is about 15 µm.

 

Fig. 5. Received optical power at the optical multimeter with and without the ORJ.

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The insertion loss of this BORJ at each wavelength point can be calculated through the received optical power with and without the ORJ, see Table 1. From Table 1, it can be concluded that the chromatic dispersion induced insertion loss is less than 0.2 dB. The bandwidth of a ORJ can be defined as the difference of the maximum wavelength and minimum wavelength where the insertion loss is 3 dB larger than its minimum value. So the available bandwidth of this BORJ at 1550 nm wavelength window is much more than 130 nm. Meanwhile, since the data rate at each wavelength is only 1.25 Gbit/s in this design, the chromatic dispersion induced inter-symbol interference (ISI) at this BORJ can be neglected.

Table 1. Insertion loss of the BORJ (dB)

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The insertion loss at 1310 nm wavelength window was tested with a broadband light source (HP 81542). The maximum insertion loss in rotation is less than 1.8 dB. The full width half maximum (FWHM) spectrum of this light source is more than 40 nm. So B_total is more than 170 nm for both wavelength windows. Currently, low-cost optical WDM modules with a channel separation of 0.8 nm have long become commercially available [20]. With the insertion loss of this BORJ less than 2 dB, a bit error rate (BER) of less than 10^-12 can be guaranteed [21,22]. Even with an effective bandwidth of 1.25 Gbit/s for the data transmission link at each wavelength [21], the total effective bandwidth of the wavelength division multiplexed ORJ in Fig. 4 is more than 200 Gbit/s. This bandwidth can meet the bandwidth requirements of the latest phase-arrayed antennas [5].

4. Space division multiplexed optical rotary joint

To further expand the available bandwidth of this BORJ and for back-compatibility with exist rotary data transmission systems where WDM technique was not adopted, a space division multiplexed ORJ was designed with a Dove prism and C-lenses, see Fig. 6. When the C-lenses of the rotator rotate at an angular speed of 2ω ₀ and the Dove prism rotates at an angular speed of ω ₀ in the same direction, the optical signals in the input channels can be continuously connected to the output channels according to the theory of geometry optics [12]. A 2:1 ratio gear train can be adopted to rotate the prism at half the speed of C-lenses [23]. Using this structure, multiple physically independent channels can be adopted in the ORJ, with each physical channel having the properties of the ORJ described in section 2 and 3.

Although this structure can accommodate more wavelength channels and has more effective bandwidth, the mechanical complexity and its high cost may counterpart its flexibility in applications where space division multiplexing is not necessary.

 

Fig. 6. Space division multiplexed BORJ using Dove prism.

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5. Conclusion

A broadband optical rotary joint was designed and fabricated using C-lenses. Its insertion loss was less than 2 dB at both the 1300 nm and 1550 nm wavelength windows. The insertion loss variation in rotation is less than 0.5 dB. WDM technique has been adopted to increase the number of optical channels. The total data transmission rate through this BORJ can be more than 200 Gbit/s. By using Dove lens, SDM technique can be adopted simultaneously with WDM technique in the design of BORJ with C-lenses. Compared to optical rotary joints implemented with diffractive optics or integrated optics, this structure of BORJ has a low cost. It can be used for transmission of both digital and analogue optical signals through relatively rotating interfaces.