**4. Comparative experimental research using a typical MWP circuit**

The previous section was referred to a comparative analysis of the VCSEL-based photodetector properties as a separate device in two modes of its operation, such as free running and optically injection locking. Continuing and expanding the study, in this section, it is considered as an element of a typical MWP circuit including, as described in the Introduction, three cascaded parts: EOC, optical processing unit (OPU), and OEC. Moreover, in this case, for completeness of coverage, three research objects are considered: a pin-photodiode, free-running VCSEL-PD, and OIL-VCSEL-PD. The general testbed for conducting experimental research is presented in **Figure 21**.

In the Figure, the EOC unit contains the tunable semiconductor laser (PurePhotonics PPCL300, 1535–1565-nm wavelength range, 6–13.5-dBm power

#### **Figure 21.**

*General testbed for conducting experimental research, where TSL, OM, OA, OFC, PD, RFA, RF VSG and RF VSA stand for tunable semiconductor laser, optical modulator, optical amplifier, optical fiber coil, photodetector, RF amplifier, RF vector signal generator, and RF vector signal analyzer, respectively. (optical connections are painted in red, electrical connections – in black).*

**189**

**Figure 22.**

*multi-position quadrature amplitude modulation.*

*Studying a LW-VCSEL-Based Resonant Cavity Enhanced Photodetector and Its Application…*

range) and the optical modulator (ThorLabs LN05S-FG, 1525–1605-nm wavelength range, 5-dB insertion loss, 35-GHz bandwidth). In addition, the OPU consists of optical amplifier (Ericsson PGE 60830, 1540–1560-wavelength range, up to 20-dB gain, 13-dBm maximum output power) and optical fiber coil (SMF-28+, 5 km). Finally, OEC unit includes PD under test (Finisar, BPDV2150, 43-GHz bandwidth, 0.6-A/W responsivity, or VCSEL-PD, or OIL-VCSEL-PD) and home-made RF amplifier (1–6-GHz operation range, 30-dB gain, 2.2-dB noise figure). Note that in all experiments, the same EOC is used with a TSL different in emitted power and wavelength, respectively. Besides, in connection with the above-mentioned structural features of the photodetectors under test, for the pin-PD a direct connection to the circuit was used, as in **Figure 21**, while VCSEL-based devices were connected to

To provide a practical focus, in the way of the experiments such an important quality indicator for a FOCS and wireless RES as an error vector magnitude (EVM), which in the first approximation is inversely proportional to the square root of the signal-to-noise ratio [25], is evaluated. To implement this measurement, using the VSG (Keysight N8152B), a digital signal at a bitrate of 120 Mbps with 16-position quadrature amplitude modulation (16-QAM) on a RF carrier in the 1–6 GHz band, is applied to the RF input of the OM and the output signal of the RFA is recorded

**Figure 22** presents the results of the comparative experiments using the optimal operating regimes that conclude from the Section 3. In the same graph, the horizontal dashed line marks the standard limit for transmitting 16-QAM signals in the emerging 5th generation cellular telecommunications network [26]. For the best vision, there are two insets in the Figure showing constellation diagrams in specific points.

• When using any of three variants of the photodetector under study, the EVM levels, which determines the quality of signal processing in the MWP circuit,

• The smallest values of this parameter, i.e. the best processing quality, and independence from the frequency of the RF carrier are obtained when using pin-photodiode, which is determined by its above mentioned bandwidth up to 43 GHz.

*Study of signal quality for a typical microwave-photonics circuit when processing high-speed data with* 

*DOI: http://dx.doi.org/10.5772/intechopen.95560*

using the VSA (Keysight M9421A).

the circuit through an optical circulator (see **Figure 12**).

The following outcomes can be drawn from this Figure.

turned out to be significantly lower than the threshold value.

#### *Studying a LW-VCSEL-Based Resonant Cavity Enhanced Photodetector and Its Application… DOI: http://dx.doi.org/10.5772/intechopen.95560*

range) and the optical modulator (ThorLabs LN05S-FG, 1525–1605-nm wavelength range, 5-dB insertion loss, 35-GHz bandwidth). In addition, the OPU consists of optical amplifier (Ericsson PGE 60830, 1540–1560-wavelength range, up to 20-dB gain, 13-dBm maximum output power) and optical fiber coil (SMF-28+, 5 km). Finally, OEC unit includes PD under test (Finisar, BPDV2150, 43-GHz bandwidth, 0.6-A/W responsivity, or VCSEL-PD, or OIL-VCSEL-PD) and home-made RF amplifier (1–6-GHz operation range, 30-dB gain, 2.2-dB noise figure). Note that in all experiments, the same EOC is used with a TSL different in emitted power and wavelength, respectively. Besides, in connection with the above-mentioned structural features of the photodetectors under test, for the pin-PD a direct connection to the circuit was used, as in **Figure 21**, while VCSEL-based devices were connected to the circuit through an optical circulator (see **Figure 12**).

To provide a practical focus, in the way of the experiments such an important quality indicator for a FOCS and wireless RES as an error vector magnitude (EVM), which in the first approximation is inversely proportional to the square root of the signal-to-noise ratio [25], is evaluated. To implement this measurement, using the VSG (Keysight N8152B), a digital signal at a bitrate of 120 Mbps with 16-position quadrature amplitude modulation (16-QAM) on a RF carrier in the 1–6 GHz band, is applied to the RF input of the OM and the output signal of the RFA is recorded using the VSA (Keysight M9421A).

**Figure 22** presents the results of the comparative experiments using the optimal operating regimes that conclude from the Section 3. In the same graph, the horizontal dashed line marks the standard limit for transmitting 16-QAM signals in the emerging 5th generation cellular telecommunications network [26]. For the best vision, there are two insets in the Figure showing constellation diagrams in specific points.

The following outcomes can be drawn from this Figure.


#### **Figure 22.**

*Study of signal quality for a typical microwave-photonics circuit when processing high-speed data with multi-position quadrature amplitude modulation.*

*Light-Emitting Diodes and Photodetectors - Advances and Future Directions*

**4. Comparative experimental research using a typical MWP circuit**

*Small-signal frequency characteristic of the OIL-VCSEL-PD under test.*

In the Figure, the EOC unit contains the tunable semiconductor laser (PurePhotonics PPCL300, 1535–1565-nm wavelength range, 6–13.5-dBm power

*General testbed for conducting experimental research, where TSL, OM, OA, OFC, PD, RFA, RF VSG and RF VSA stand for tunable semiconductor laser, optical modulator, optical amplifier, optical fiber coil, photodetector, RF amplifier, RF vector signal generator, and RF vector signal analyzer, respectively. (optical* 

*connections are painted in red, electrical connections – in black).*

The previous section was referred to a comparative analysis of the VCSEL-based photodetector properties as a separate device in two modes of its operation, such as free running and optically injection locking. Continuing and expanding the study, in this section, it is considered as an element of a typical MWP circuit including, as described in the Introduction, three cascaded parts: EOC, optical processing unit (OPU), and OEC. Moreover, in this case, for completeness of coverage, three research objects are considered: a pin-photodiode, free-running VCSEL-PD, and OIL-VCSEL-PD. The general testbed for conducting experimental research is

**188**

**Figure 21.**

presented in **Figure 21**.

**Figure 20.**

