Introduction
In recent years, the integrated optical passive devices are smaller in size and more mature in technology, occupying a considerable part of the market share. As one of the key devices of optical communication, erbium-doped fiber amplifier (EDFA) has become the technical focus of competition among many manufacturers due to its integration, miniaturization, multi-function and low cost. The integration of hybrid optical passive devices is not to use integrated technology to make devices, but to integrate separate devices together. The above competitive advantages of EDFA can be realized by integrating optical isolator, wavelength division multiplexing (WDM) devices, optical circulator and test access port (TAP) splitter into a hybrid device. At the same time, the manufacturing process of hybrid optical passive devices is also one of the key factors to realize the competitive advantage of the above-mentioned EDFA technology.
When optical signal is transmitted in optical fiber, it usually has some loss. Therefore, EDFA is needed to amplify the light in the optical fiber network to compensate for the energy loss caused by insertion loss. As a very important optical amplifier in the medium and long-distance optical network, EDFA has three applications in the optical network, including relay amplification, pre amplification and post amplification, so as to ensure that optical signals can transmit hundreds, even thousands of kilometers. In addition, EDFA is also used in lidar and datacenter.
Based on the miniaturization of EDFA, the principle and application of hybrid devices are introduced. Figure 1 is the schematic diagram of the EDFA module. It can be seen that the output and input terminals of the optical module all need the coupler, isolator and optical coupler with small splitting ratio. We call the integrated device at the input end as the front type (generally including isolator, WDM and coupler), and the integrated device at the output end as the post type (generally including isolator and WDM).
A typical on-line optical isolator is shown in Fig. 2, which consists of four main components: magnetic tube, Faraday rotator, half wave plate (HWP), two LiNbO3 wedge (LN wedge), and a pair of fiber collimators. As shown in Fig. 2 (a), after passing through the isolator core, the parallel beam emitted from the collimator is divided into two ordinary-beams (O beam) and extraordinary-beams (E beam), which are coupled to the output fiber port through the receiving collimator. As shown in Fig. 2 (b), after passing through the isolator core, the parallel beam emitted from the out end is still separated into two ordinary-beams and extraordinary-beams. However, at this time, ordinary-light will become extraordinary-ray, and extraordinary-light will become ordinary-light. Because ordinary-light and extraordinary-light are separated, input collimator can not couple these two beams into optical fiber, thus playing the role of reverse isolation of light.
Part 1: Optical isolation and gain flat filter (GFF) hybrid device
Optical isolation and gain flat filter (GFF) hybrid device shown as Fig.3. Input signal beam is collimated before isolator core. The isolation core mainly realizes the function of isolating reverse optic as a protection. After isolator core, there is a filter. The filter plays role as a gain flat filter (GFF). It plates thin film. The spectrum gain of GFF is reverse to the spectrum gain of signal. This hybrid device mainly applied into EDFA and Raman amplifiers. Its main features include: it belongs to integrated device; compact size; accurate matching gain distribution; low error function; no epoxy resin adhesive on the light path; Telcordia compatible; RoHS compatible.
This hybrid device mainly applied into EDFA. It can make coupler or WDM device hybrid together. The integrated device not only decrease size of module, but also realize the function of isolation, wavelength multiplex/de-multiplex and gain flat filter. The main specifications of hybrid devices of optical isolator and gain flattening filter include: wavelength, insert loss, peak to peak error function, isolation, return loss, polarization dependent loss, polarization mode dispersion, directivity, carrying power and package size.
Part 2: 1×2 WDM and optical isolation hybrid device (IWDM)
As shown in Fig. 4, the function diagram of 1×2 WDM and isolator hybrid device is shown. It consists of two fiber collimator with tilted end face, tunable wavelength filter (TFF) realized by thin film technology, isolator and single fiber collimator with tilted end face. A beam of broadband light (as shown by the black arrow) incident from fiber 1 is collimated by graded index lenses (G-Lens), and the beam becomes parallel. TFF wave plate is close to the exit end face of G-LENS, and only one central wavelength is allowed to pass through, and other wavelengths will be reflected. Generally, the passband bandwidth of TFF filter is 20nm. The TFF chip selects a certain wavelength and transmits it (as shown by the green arrow) and outputs from the fiber 3 port. At the same time, the other wavelengths of light (as shown by the red arrow) are reflected back to the fiber 2 port by the TFF wave plate.
The 1×2 WDM and optical isolation hybrid device (IWDM) can be used in EDFA with high isolation requirements, which can not only ensure the optical isolation to meet the requirements, but also reduce the size of the whole EDFA module. The pump light can also be incident into erbium-doped fiber (EDF) from the fiber 2 port, thus playing the role of optical amplification, as shown in Fig. 1. The isolator plays a role of reverse isolation, which further ensures that the isolation of WDM devices meets the requirements of EDFA.
Part 3: 2×4 WDM and optical isolation hybrid device (IWDM)
As shown in Fig. 5, the function diagram of the 2×4 WDM and isolator hybrid device is shown. Using two TFF filters with different central wavelengths, the function of two 1×2 WDM and isolator hybrid devices can be realized. A four fiber collimator and a dual fiber collimator are used in this device. The optical fiber end faces are tilted, so that the return loss value can meet the requirement of more than 60 dB. Due to the use of four fiber collimator, the coupling process is required to be higher. This hybrid device can make the function of pump 1480nm wavelength reversely from port 4th, and monitor input optic signal simultaneously from the port 2nd.
The device consists of a four fiber collimator, a dual fiber collimator, an isolator and two TFF filters with different central wavelengths. The pump light can be incident into erbium-doped fiber (EDF) from port 2 and port 4 respectively, which plays the role of optical amplification. With two input ports of pump light, different wavelengths of pump light can be input at the same time to meet the flexible requirements of EDFA. The size of this device is obviously smaller than that of two separate 1×2 WDM and isolator hybrid devices. It can be used in EDFA modules with higher requirements and further reduce its size.
Part 4: 2×2 WDM-Isolation-TAP hybrid device (WDM-ISO-TAP)
Test access port (TAP) / traffic access port (TAP) is a new type of integrated equipment used for network monitoring. It does not need to provide power supply inside. It belongs to passive optical network equipment. It provides a method to obtain network traffic under the condition of constant normal network traffic. Therefore, TAP can be used to monitor network performance, solve network failure, analyze network traffic and prevent malicious attacks.
As shown in Fig. 6, it is a 2×2 WDM device with a hybrid of isolators and TAP splitters. Port 1 is connected to the erbium-doped fiber (EDF) side. The input beam passes through a double fiber collimator and the beam is collimated. The collimated beam passes through the double isolator core, and the beam splitting prism divides the beam into two parts, one of which has 99% power and the other with 1% power, and enters the 3 and 4 ports of the dual fiber collimator respectively.
The optical path of signal light in the device is shown in Fig. 7. The signal light comes out from the dual fiber collimator at a certain off-axis angle. When passing through the isolator, the signal light will shift x mm laterally. After entering the beam splitter prism, it will be divided into two beams, 1% of which will move upward y mm, and 99% of which will remain unchanged. In this way, two parallel beams with a power ratio of 99:1, a distance of Y and an included angle of Z degree are emitted from the beam splitter prism, which can be coupled into a double fiber collimator.
Generally, WDM-ISO-TAP hybrid device is used in the backward pump part of EDFA, which is difficult to realize. WDM-ISO-TAP hybrid device can be used in the backward pump part of EDFA. It has the advantages of small volume and low cost, and the optical path is easy to realize.
Part 5: Ultra hybrid mini OA component (UHMC)
In the direction of miniaturization and integration of optical devices, the concept of all in one is proposed in the industry. A series of ultra small and highly integrated optical amplifier (OA) passive devices are exhibited at China International Optoelectronic Exposition 2018 (CIOE2018), which provides device level solutions for small / ultra small EDFA. The function diagram is shown in Fig-8(a). The five in one ultra hybrid mini OA component (UHMC) integrates the function of isolator + 1550 / 980 WDM + isolator + coupler + PD in a φ3.5 * 25mm device. One device can solve the passive device scheme of EDFA.
As shown in Fig. 8(b), the device part 1 is the schematic diagram of a optical isolator core. The light beam enters from the collimator, passes through wedge angle plate, Faraday plate and wedge angle plate, and is divided into ordinary light (O light) and extraordinary light (E light). Because the separation distance between O beam and E beam is very small in forward incidence, it can be coupled into collimator output fiber normally. Due to the role of the isolator core, the separation distance between the reverse O-beam and E-beam is relatively large, which can not be coupled into the input fiber of collimator normally, so as to achieve the isolation effect.
The second part of the device is a 1 x 2 WDM device schematic diagram. The signal beam incident from the isolator is collimated by a graded refractive index lens. The output end of the lens is coated with a tunable wavelength filter (TFF) realized by thin film technology. The output end face of the filter plate is coated with a specific reflection film, which allows the specific central wavelength to pass through, and the beam with the remaining wavelength is reflected back . At the same time, the incident pump light input from the other port of the WDM device, and the pump wavelength can be 980 nm. The device 4 uses a beam splitting prism to divide the beam into two beams with a power ratio of 99:1. 1% of the light is output to the PD terminal for monitoring and 99% of the light is output as signal light.
Using the hybrid design, it can realize the same function with less collimators. The cost will be reduce, and the device size will be smaller. As shown in Fig.7, the input and output collimators are necessary, and without other collimators in the five in one ultra hybrid mini OA component (UHMC) .
Reference
- Zhujun Wan; Mingcui Cao; Fengguo Luo,etc. Semiconductor optoelectronics,2002.
Written by Dongtang Yang HYC Co., Ltd
Optical Passive Components 