Cost-Effective Half-Duplex Transceivers for Fiber-to-the-Room Network Enabled by Optical Feedback Resistant Single-Mode Lasers

We propose and demonstrate a cost-effective half-duplex transceiver for Fiber-to-the-Room (FTTR) network by using a homemade uncooled and isolator-free distributed feedback (DFB) laser as both receiver (Rx) and transmitter (Tx). AlGaInAs multiple quantum wells (MQWs) and a partially corrugated grating are utilized for uncooled and isolator-free operation of the DFB lasers. Tx/Rx mode switching is implemented by a specially designed complementary metal-oxide-semiconductor (CMOS) chip including biasing circuits, a trans-impedance amplifier (TIA) and a single pole double throw (SPDT) radio frequency (RF) switch. The transceiver can work up to 5 Gbps in both Tx and Rx modes with 17 ns Rx to Tx switching time and 94.4 ns Tx to Rx switching time, respectively. The Tx output optical power of the transceiver is 11 dBm, while the Rx sensitivity is −18 dBm at a bit error rate (BER) of 10−3. This integration method proves that the transceiver is a cost-effective high speed light source for the future FTTR network.


I. INTRODUCTION
W ITH the rapid increase of amount of mobile terminals and innovative service applications, such as ultra-highdefinition video service, cloud virtual reality(VR), cloud gaming, online education, and telecommuting, the bandwidth for access networks continues to migrate upwards [1], [2], [3]. As an evolution version of Fiber-to-the-Home (FTTH), FTTR is blooming globally, further extending fiber to every room with the help of 10G passive optical network (PON) and Wi-Fi 6 to increase throughput in the home network and office space [4], [5], [6], [7]. However, in the FTTR network based on PON architecture and protocol, as shown in Fig. 1(a) [8].
Wi-Fi interference and resource conflicts are also challenging to resolve. Therefore, a cost-friendly FTTR architecture based on Radioover-Fiber (RoF) system is proposed, as shown in Fig. 1(b). In this architecture, Wi-Fi radio-frequency (RF) signals are generated from the primary ONT and extended to each edge ONT through an optical fiber. Unified dynamic Wi-Fi management is possible on the centralized primary ONT to eliminate Wi-Fi interference and resource conlicts that are difficult to resolve in traditional PON architecture. Moreover, the PHY hardware architecture of an edge ONT becomes simplified, which includes only an optical transceiver and a Wi-Fi RF front end module (FEM). No Wi-Fi MAC/PHY are needed for edge ONTs. Unlike traditional full-duplex PON transceivers with different wavelengths for the Tx and Rx paths, the transceivers in this architecture operate in a single-wavelength half-duplex approach. By reusing Tx and Rx optics without Wi-Fi signal coding function on the edge ONT, the optical transceiver cost of edge ONTs is expected to be reduced by half, which is very attractive for the mass deployment of FTTR network. Importantly, the introduction of half-duplex optical transceiver does not cause a total bandwidth loss, since the Wi-Fi access point (AP) in each room of the FTTR network also operates in half-duplex mode [9], [10]. However, there are no such demonstrated system including optical transceivers with fast configuration to meet the requirements of the future FTTR network, so far.
In this work, we demonstrate a high-speed cost-effective halfduplex transceiver for the future FTTR network. It employs a TO-CAN packaged bi-directional optical subassembly (BOSA) module with a homemade high optical feedback resistant DFB laser chip as both Tx and Rx, a specially designed CMOS chip including biasing circuits, a TIA and an SPDT RF switch for dynamic Tx and Rx transition. 5 Gbps transmission for both Tx and Rx has been experimentally demonstrated.

II. DESIGN AND FABRICATION OF ISOLATOR-FREE DFB LASERS FOR THE HALF-DUPLEX TRANSCEIVER
In principle, a DFB laser diode can be used as an optical receiver in the low-cost half-duplex optical communication system when it is reverse biased because of its PIN structure nature. The laser-based receiver's electrical bandwidth is much larger than the LED receiver's due to its small chip size, which is essential for high-speed FTTR applications. Therefore, we use a DFB laser as both Tx and Rx of the transceiver. Similar to the application of LED in visible light communication [11], [12], [13], [14], light is bidirectional to the laser chip with no optical isolator assembled. Unlike LEDs, DFB lasers with high modulation rate are more sensitive to light reflections [15], [16]. In order to eliminate isolators, single-mode lasers highly tolerant to optical feedback are indispensable.
It is known that the DFB lasers with gain/complex-coupled or partial grating are highly tolerant to optical feedback in comparison with DFB lasers with conventional uniform index-coupled grating [17], [18], [19], [20]. However, to the best of our knowledge, the modulation speed of the optical feedback resistant DFB lasers is limited to 2.5 Gbps. In this article, we use partial grating for the isolator-free 10 Gbps DFB lasers. By optimizing the κL factor of the grating without significantly lowering the slope efficiency, optical feedback resistance is further improved. The coupling coefficient κ is mainly determined by the optical confinement factor as well as the refractive index contrast of the grating layer. In order to improve the coupling coefficient κ, the thickness and refractive index of the grating layer, the refractive index of the capping layer, as well as the distance between the grating layer and the active layer of the lasers, can all be optimized.
To realize uncooled operation at 10 Gbps in a wide temperature, AlGaInAs multiple quantum wells (MQWs) are used as the active region of the DFB lasers because of their better' and large differential gain that arises from a larger conduction band offset compared with conventional InGaAsP MQWs widely used for optoelectronic devices [21], [22], [23]. Optical confinement is also provided by two separate confinement layers located below and above the active region, respectively. For the fabrication of the device, the MQWs layer sandwiched by the lower/upper  separate confinement heterojunction layers, and the grating layer are first grown on an n-InP substrate by metal-organic chemical vapor deposition (MOCVD). After the grating fabrication and MOCVD regrowth to the p-InP cladding layer and P + -InGaAs contact layer. The ridge waveguide was formed by wet etching. A Ti-Au metal layer is sputtered on the p-InGaAs contact layer to form the p-electrode. After the substrate is thinned, Au-Ge-Ni metal is evaporated on the backside. Finally, chips are cleaved, and antireflection and high-reflection coatings are applied to the respective facets. The cavity length is designed to be about 200 µm to obtain high relaxation oscillation frequency in the temperature range from −25°C to 85°C. Fig. 2 shows the measured light output power versus injection current curve of a typical DFB under a continuous wave (CW) condition. The threshold currents and the slope efficiencies are 8.5 mA and 0.46 W/A at 25°C, and 16.2 mA and 0.30 W/A at 85°C, respectively. Stable single-mode operation with side mode suppression ratio of more than 45 dB is achieved from −25°C to 85°C. Then, the dynamic performance of the DFB laser is characterized. Fig. 3 depicts the eye diagrams of the directly modulated DFB laser (DML) operated at 10 Gbps. The pseudo random binary sequence (PRBS) length is 2 31 −1. The DML shows a clear eye opening with an extinction ratio of around 7dB from −25°C to 85°C, indicating that the DFB laser is effective for uncooled operation.
To evaluate the optical feedback tolerance of the fabricated DFB laser, the experimental setup is similar to that in Ref. [18] is Authorized licensed use limited to the terms of the applicable license agreement with IEEE. Restrictions apply. built. Optical feedback is carried out to the laser through a 3 dB coupler and an optical circulator. The optical feedback ratio is controlled by a variable optical attenuator, and the polarization of the feedback light is adjusted by a polarization controller to obtain the maximum feedback effect at a given feedback level to simulate the worst case of the laser. Fig. 4 summarizes the backto-back BER performance of the laser at 10Gbps with different optical feedback levels. The power penalty with a BER of 10 −3 is less than 1 dB under −12 dB external optical feedback even at 85°C.These experimental results clearly demonstrate that the laser can operate without expensive isolators and thermoselectric coolers (TEC).

III. PACKAGE AND MEASUREMENT OF HALF-DUPLEX TRANSCEIVER
The isolator-free DFB laser is co-packaged into TO-CAN header with a specially designed CMOS chip, including a TIA and an SPDT RF switch, as shown in Fig. 5. The integrated CMOS chip is used to change the bias condition of the DFB laser by tuning the controlled voltage of the SPDT. The DFB laser works as a transmitter with a forward biasing current, while it works as a receiver with a reverse biased voltage. TIA is used to improve the Rx performance. The DFB chip, CMOS chip, RF substrate, and lens are assembled on the base of TO-CAN. The DFB laser and CMOS chip are directly connected by gold wires.
This low-cost coaxial-type package proved to be able to transmit a 10 Gbps signal [24]. To reduce the RF resonance, the lead of the TO header is cut as short as possible. A pair of lenses are used for better optical coupling between laser/photodiode and optical fiber. The lens on the base is used to collimate the light from the laser, while another lens in the housing of the transceiver is used to focus the light into the optical fiber.
The responsivity of the 1270 nm DFB laser at a reserved bias of 3.3 V is around 0.7 A/W, while the O/E bandwidth of  the laser is around 4 GHz. The switching performance of the SPDT is evaluated by measuring the delay of the slowest bias voltage node versus the switching control signal. As shown in Fig. 6, the half-duplex operation is demonstrated with an Rx to Tx switching time of 17 ns and a Tx to Rx switching time of 94.4 ns, respectively. This fast switching helps to reduce the overhead time of Tx and Rx burst packets in half-duplex communication, resulting in a high payload capacity.
To evaluate the performance of the receiver, the DFB laser is reverse biased at −2.6 V. NRZ optical signal with a sequence length of 2 31 −1 and a dynamic extinction ratio of 8 dB coupled to the transceiver through the lucent connector (LC). Fig. 7(a) shows the eye diagram of the device at the bit rate of 5 Gbps. Clearly opened eye diagram is obtained. In addition, the eye diagram aperture also suggests that higher bit-rate signal detection is potentially achievable. Fig. 7(b) shows the back-to-back BER results of the receiver. We can see a small polarization dependence of the receiver. The receive sensitivity at BER of  10 −3 is −20.6 dBm and −18 dBm at 5 Gbps with TE and TM polarized light at 1270 nm, respectively. This small polarization dependence is due to the intrinsic structure of the laser and can be reduced by increasing the bias voltage. It is supposed that the receiver sensitivity is high enough for FTTR network. We also measure the Tx performance of the transceiver. In Tx mode, the DC bias current and the AC modulation current are injected into the DFB laser via an off-chip Bias-Tee. The output optical power of the Tx is around 11 dBm at an average injection current of 60 mA. The measured eye diagrams in Tx mode are shown in Fig. 8. Clearly opened eye diagrams are obtained with the dynamic extinction ratio of 6.6 dB at the bit rate of 5 Gbps and 10 Gbps, respectively. When the half-duplex optical transceiver is used as both the Rx and Tx in the network, simultaneously, the optical link budget for 5 Gbps is calculated to be more than 29 dB. These experimental results indicate that the transceiver is a costeffective high speed light source for the future FTTR network.

IV. CONCLUSION
In summary, we have demonstrated a low cost half-duplex optical transceiver for the future FTTR network with a fast turnaround time enabled by a homemade isolator-free uncooled distributed feedback (DFB) laser as both receiver and transmitter. Half-duplex operation of the transceiver is demonstrated with an Rx to Tx switching time of 17 ns and a Tx to Rx switching time of 94.4 ns, respectively. More than 29 dB optical link budget at 5 Gbps is reached when this half-duplex optical transceiver is used as both the Rx and Tx in the system, simultaneously. This integration method proves that the transceiver is a cost-effective high speed light source for the future FTTR network. Due to its cost advantage, the half-duplex transceivers are believed to play a huge role in the future FTTR network with the access bit rate exceeding gigabit.