**3. UWB signal modulation**

**Figure 11.** The PIC layout of the on-chip UWB generation system. The inset are the microscope images of the microrings, photodetector, and long waveguides.

UWB-IR is a wide band RF spectrum with extremely low power spectral density. It actually functions as a wireless carrier to deliver information in a short reach scenario. The modulation of UWB-IR is also a fundamental issue in the transmitter design. One of the most intuitive modulation formats that can apply for UWB-IR is the on-off keying (OOK). This format is easy to implement, but it suffers from intensity noise. Pulse position modulation and biphase modulation are alternative formats apart from OOK. Here, we introduce a novel UWB signal modulation scheme using the tunable filter, which can be compatible for both OOK and biphase modulation.

In **Figure 12**, we illustrate the working principle of how to generate and modulate the UWB monocycle pulse using a single tunable filter. Such a tunable filter should have a large extinction ratio and fast tuning speed. Electrooptic silicon microring is one of the perfect candidates for this scheme. As shown in **Figure 12(a)**, when the laser wavelength is set at the center of the linear slope of the ring resonance, a monocycle pulse with a certain intensity is generated. When a driving voltage is applied to the ring resonator, the ring resonance is shifted and its resonance dip is shifted to match the laser wavelength. As a result, the pulse will vanish due to the low intensity at the ring resonance dip and the OOK modulation is achieved.

realized if the driving voltages are well controlled with equal intensity for the polarity-

**Figure 13.** (a) The top-view schematic diagram of the silicon microring modulator and (b) the schematic diagram of the

**Figure 12.** The schematic principle of the tunable-filter-based (a) UWB OOK modulation and (b) UWB biphase

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An active microring resonator (microring modulator) can be used to simultaneously generate and modulate the UWB-IR. The top view and cross section schematic structure of a silicon ring modulator are shown in **Figure 13(a)** and **(b)**. The resonance wavelength tuning is

*neffL* = *m <sup>i</sup>* (4)

achieved by modifying the resonance condition, which is expressed by

reverted monocycle pulse.

P-N silicon waveguide.

modulation.

If the driving voltage is larger, a larger wavelength shift will be introduced to the ring resonance as shown in **Figure 12(b)**. First, the laser wavelength is set to a midpoint at one side of the resonance slope center. Without the electrooptic tuning, a monocycle pulse will be generated. By applying an appropriate swing voltage, the laser wavelength can be just located to the other side of the resonance slope. As a result, the polarity of the monocycle pulse will be reverted since it experiences a π phase shift. The biphase modulation can be

**Figure 12.** The schematic principle of the tunable-filter-based (a) UWB OOK modulation and (b) UWB biphase modulation.

**Figure 11.** The PIC layout of the on-chip UWB generation system. The inset are the microscope images of the microrings,

UWB-IR is a wide band RF spectrum with extremely low power spectral density. It actually functions as a wireless carrier to deliver information in a short reach scenario. The modulation of UWB-IR is also a fundamental issue in the transmitter design. One of the most intuitive modulation formats that can apply for UWB-IR is the on-off keying (OOK). This format is easy to implement, but it suffers from intensity noise. Pulse position modulation and biphase modulation are alternative formats apart from OOK. Here, we introduce a novel UWB signal modulation scheme

In **Figure 12**, we illustrate the working principle of how to generate and modulate the UWB monocycle pulse using a single tunable filter. Such a tunable filter should have a large extinction ratio and fast tuning speed. Electrooptic silicon microring is one of the perfect candidates for this scheme. As shown in **Figure 12(a)**, when the laser wavelength is set at the center of the linear slope of the ring resonance, a monocycle pulse with a certain intensity is generated. When a driving voltage is applied to the ring resonator, the ring resonance is shifted and its resonance dip is shifted to match the laser wavelength. As a result, the pulse will vanish due

If the driving voltage is larger, a larger wavelength shift will be introduced to the ring resonance as shown in **Figure 12(b)**. First, the laser wavelength is set to a midpoint at one side of the resonance slope center. Without the electrooptic tuning, a monocycle pulse will be generated. By applying an appropriate swing voltage, the laser wavelength can be just located to the other side of the resonance slope. As a result, the polarity of the monocycle pulse will be reverted since it experiences a π phase shift. The biphase modulation can be

using the tunable filter, which can be compatible for both OOK and biphase modulation.

to the low intensity at the ring resonance dip and the OOK modulation is achieved.

photodetector, and long waveguides.

**3. UWB signal modulation**

86 UWB Technology and its Applications

**Figure 13.** (a) The top-view schematic diagram of the silicon microring modulator and (b) the schematic diagram of the P-N silicon waveguide.

realized if the driving voltages are well controlled with equal intensity for the polarityreverted monocycle pulse.

An active microring resonator (microring modulator) can be used to simultaneously generate and modulate the UWB-IR. The top view and cross section schematic structure of a silicon ring modulator are shown in **Figure 13(a)** and **(b)**. The resonance wavelength tuning is achieved by modifying the resonance condition, which is expressed by

$$m\_{\text{eff}}L = \, ^\circ \kappa \lambda \tag{4}$$

where neff is the waveguide effective refractive index, L is the round-trip length, m is an integer, and λ<sup>i</sup> is the resonance wavelength. The cavity waveguides are normally doped with P- and N-type implantations as shown in **Figure 13**. Silicon is a semiconductor; the doping improves the conductivity of the waveguides, which introduces the free carriers inside the waveguides. The P- and N-type doping forms a P-N junction inside the waveguide with several hundreds of nanometers width. By electrical tuning, the effective index of the waveguide changes due to the variations in carrier distribution, which is called free carrier dispersion effect [32]. The index change results in the resonance shift indicated in Eq. (4), and thus, it could be utilized for UWB-IR generation and modulation.

#### **4. UWB-over-fiber**

UWB-over-fiber (UWBoF) has been proposed to effectively distribute UWB-IR signals while keeping the wireless transmission within tens of meters range. For such optical distribution, it can be more cost-effective to implement the generation and distribution of UWB-IR in optical domain, thus avoiding the need for multiple electrical to optical to electrical conversions. An interesting idea is to integrate the UWB signal distribution networks into the existed access networks. By sharing the same fiber, optical UWB-IR and wired downstream signal coexist in the access fiber networks. The fiber to the home brings the wireless UWB signal to the home as well. There are many UWBoF architectures discussed in the literatures. Among those access network solutions, wavelength division multiplexed-passive optical network (WDM-PON) is one of the most promising systems as it has extremely large data capacity. Though it has not been commercialized yet due to the cost issues, it has a bright future since it can fully utilize the nearly infinite optical bandwidth. Here, we introduce a hybrid solution of UWB-IR wireless service that is integrated with a WDM-PON. It provides a wired baseband data service and a UWB-IR signal distributed from the other remote antenna unit (RAU).

a grating coupler. The on-chip microring resonator performs the PM-IM conversion. The UWB monocycle pulse is generated and converted to electrical signal by a waveguide germanium photodetector, which can be fabricated using fully CMOS compatible process. If the photodetector has enough responsivity, its output can be directly fed into the antenna

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**Figure 14.** The schematic diagram of the UWBoF system, which is compatible with WDM-PON.

Though UWB has not been so widely implemented as Wi-Fi, Bluetooth, and other narrowband services, its broadband nature and intrinsic advantages make it extremely suitable for some particular applications like indoor positioning and tracking. The UWB service has much higher precision and less interferences than other radio systems, which make it promising for Internet of things in the near future. MWP is emerging as an alternative technology for other UWB signal generation and processing in electrical domain. Over the past decade, we have witnessed a lot of lab demonstrations of UWB photonics like UWB waveform generation, UWB signal modulation, UWB-over-fiber, and so on. There are many other topics that remain open to the whole community such as the study on impact of fiber channel impairments, integration of UWB antenna and photonic chip, more power-efficient UWB pulse generation,

advanced formats of UWB signal modulation, and so on.

Address all correspondence to: kxu@hit.edu.cn Harbin Institute of Technology, Shenzhen, China

for wireless emission.

**5. Conclusion**

**Author details**

Ke Xu

The schematic system configuration is shown in **Figure 14**. In such a WDM distribution system that integrates the UWB-IR and wired baseband signal, a silicon PIC is implemented in an optical network unit (ONU) within the WDM-PON architecture. The silicon chip serves as a wireless access point and a wired signal receiver. Centralized light sources are located at the central office, and there are two laser diodes with wavelength close to each other for each transmitter (Tx). This is a trick that is used for separation of the UWB and wired signal at the ONU side. For Tx1, laser diode1 (LD1) is intensity modulated with wired signal, while LD2 is phase modulated with the UWB wireless data. The wavelengths of LD1 and LD2 occupy two adjacent channels of the AWG. All the channels are multiplexed and transmitted in the distribution fiber. At the remote node, the wavelength channel allocated to wired signal is sent to the receiver directly. The other channel is separated into two branches with a portion of the signal remodulated by the upstream data. Since the UWB signal is a phasemodulated CW, the phase information will not affect the upstream data if intensity modulation format is used. Another part of the signal is coupled onto the silicon chip through

**Figure 14.** The schematic diagram of the UWBoF system, which is compatible with WDM-PON.

a grating coupler. The on-chip microring resonator performs the PM-IM conversion. The UWB monocycle pulse is generated and converted to electrical signal by a waveguide germanium photodetector, which can be fabricated using fully CMOS compatible process. If the photodetector has enough responsivity, its output can be directly fed into the antenna for wireless emission.

### **5. Conclusion**

where neff is the waveguide effective refractive index, L is the round-trip length, m is an integer,

UWB-over-fiber (UWBoF) has been proposed to effectively distribute UWB-IR signals while keeping the wireless transmission within tens of meters range. For such optical distribution, it can be more cost-effective to implement the generation and distribution of UWB-IR in optical domain, thus avoiding the need for multiple electrical to optical to electrical conversions. An interesting idea is to integrate the UWB signal distribution networks into the existed access networks. By sharing the same fiber, optical UWB-IR and wired downstream signal coexist in the access fiber networks. The fiber to the home brings the wireless UWB signal to the home as well. There are many UWBoF architectures discussed in the literatures. Among those access network solutions, wavelength division multiplexed-passive optical network (WDM-PON) is one of the most promising systems as it has extremely large data capacity. Though it has not been commercialized yet due to the cost issues, it has a bright future since it can fully utilize the nearly infinite optical bandwidth. Here, we introduce a hybrid solution of UWB-IR wireless service that is integrated with a WDM-PON. It provides a wired baseband data service and a UWB-IR signal distributed from the other remote

The schematic system configuration is shown in **Figure 14**. In such a WDM distribution system that integrates the UWB-IR and wired baseband signal, a silicon PIC is implemented in an optical network unit (ONU) within the WDM-PON architecture. The silicon chip serves as a wireless access point and a wired signal receiver. Centralized light sources are located at the central office, and there are two laser diodes with wavelength close to each other for each transmitter (Tx). This is a trick that is used for separation of the UWB and wired signal at the ONU side. For Tx1, laser diode1 (LD1) is intensity modulated with wired signal, while LD2 is phase modulated with the UWB wireless data. The wavelengths of LD1 and LD2 occupy two adjacent channels of the AWG. All the channels are multiplexed and transmitted in the distribution fiber. At the remote node, the wavelength channel allocated to wired signal is sent to the receiver directly. The other channel is separated into two branches with a portion of the signal remodulated by the upstream data. Since the UWB signal is a phasemodulated CW, the phase information will not affect the upstream data if intensity modulation format is used. Another part of the signal is coupled onto the silicon chip through

 is the resonance wavelength. The cavity waveguides are normally doped with P- and N-type implantations as shown in **Figure 13**. Silicon is a semiconductor; the doping improves the conductivity of the waveguides, which introduces the free carriers inside the waveguides. The P- and N-type doping forms a P-N junction inside the waveguide with several hundreds of nanometers width. By electrical tuning, the effective index of the waveguide changes due to the variations in carrier distribution, which is called free carrier dispersion effect [32]. The index change results in the resonance shift indicated in Eq. (4), and thus, it could be utilized

and λ<sup>i</sup>

for UWB-IR generation and modulation.

**4. UWB-over-fiber**

88 UWB Technology and its Applications

antenna unit (RAU).

Though UWB has not been so widely implemented as Wi-Fi, Bluetooth, and other narrowband services, its broadband nature and intrinsic advantages make it extremely suitable for some particular applications like indoor positioning and tracking. The UWB service has much higher precision and less interferences than other radio systems, which make it promising for Internet of things in the near future. MWP is emerging as an alternative technology for other UWB signal generation and processing in electrical domain. Over the past decade, we have witnessed a lot of lab demonstrations of UWB photonics like UWB waveform generation, UWB signal modulation, UWB-over-fiber, and so on. There are many other topics that remain open to the whole community such as the study on impact of fiber channel impairments, integration of UWB antenna and photonic chip, more power-efficient UWB pulse generation, advanced formats of UWB signal modulation, and so on.

### **Author details**

Ke Xu Address all correspondence to: kxu@hit.edu.cn Harbin Institute of Technology, Shenzhen, China

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