**3. VCSEL-based photodetector (VCSEL-PD)**

Following the detailed analysis of the object for investigation carried out in Section 2, in this section we will conduct a comparative study of the typical characteristics of an inherent pin-photodetector and a photodetector based on a LW-VCSEL in two modes of its operation: in free-running mode and in OIL mode.

#### **3.1 Reverse-biased free-running mode**

**Figure 12** depicts a common testbed for measuring static and dynamic characteristics of the VCSEL-PD using typical in photodetector's characterizing techniques and procedures [1, 3]. The testbed, besides the VCSEL-PD under test, contains a set of accessories such as optical circulator (Opneti, CIR-3-1550), master laser (PurePhotonics PPCL300, 1530–1565-nm wavelength range, 6–13.5-dBm power range), optical amplifier (Ericsson PGE 60830, 1540–1560-wavelength range, up to 20-dB gain, 13-dBm maximum output power), optical modulator (ThorLabs LN05S-FG, 1525–1605-nm wavelength range, 5-dB insertion loss, 35-GHz bandwidth), bias-T (Pasternack, PE1BT-1002, 40-GHz bandwidth) as well as corresponding DC power suppliers, digital multi-meter (INSTEK GDM-8245), and measuring tools including the same apparatuses as in **Figure 4**.

**183**

**Figure 13.**

*Dark current vs. reverse voltage characteristic.*

up to 3 V.

**Figure 12.**

*black).*

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

Figures below demonstrate the results of measuring the key static and dynamic characteristics of the free-running VCSEL-PD using reverse DC bias from DC3. First, **Figure 13** presents dark current vs. reverse voltage characteristic. For the device under test, dark current value is not more than 70 nA at a reverse voltage

*Common testbed for measuring static and dynamic characteristics of the VCSEL-PD under test, where ML, OM, OA, OCL, DC, DM, OSA, OPM, and RF VNA stand for master laser, optical modulator, optical amplifier, optical circulator, DC source, digital multi-meter, optical spectrum analyzer, optical power meter, and RF vector network analyzer, respectively. (optical connections are painted in red, electrical connections – in* 

**Figure 14** presents the effect of reverse bias voltage on photocurrent of the VCSEL-PD under test at the 1560.95-nm incident optical powers of 1, 2, and 3 mW. **Figure 15** characterizes photocurrent vs. incident optical power of 1560.95-nm wavelength for the VCSEL-PD under test at reverse voltage of 1 V. As one can see from the Figure, the current responsivity at the initial segment of the characteristic is about 0.2 A/W, and the threshold of its linearity at a level of −1 dB is approximately 2.6 mW.

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

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

#### **Figure 12.**

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

**3. VCSEL-based photodetector (VCSEL-PD)**

**3.1 Reverse-biased free-running mode**

*when the power of the ML is 5 dBm (2) or 8 dBm (3).*

*Optical spectrum of the OIL-VCSEL under test.*

Following the detailed analysis of the object for investigation carried out in Section 2, in this section we will conduct a comparative study of the typical characteristics of an inherent pin-photodetector and a photodetector based on a LW-VCSEL in two modes of its operation: in free-running mode and in OIL mode.

*Small-signal modulation characteristics of OIL-VCSEL under test in free-running mode (1), and in OIL mode* 

**Figure 12** depicts a common testbed for measuring static and dynamic characteristics of the VCSEL-PD using typical in photodetector's characterizing techniques and procedures [1, 3]. The testbed, besides the VCSEL-PD under test, contains a set of accessories such as optical circulator (Opneti, CIR-3-1550), master laser (PurePhotonics PPCL300, 1530–1565-nm wavelength range, 6–13.5-dBm power range), optical amplifier (Ericsson PGE 60830, 1540–1560-wavelength range, up to 20-dB gain, 13-dBm maximum output power), optical modulator (ThorLabs LN05S-FG, 1525–1605-nm wavelength range, 5-dB insertion loss, 35-GHz bandwidth), bias-T (Pasternack, PE1BT-1002, 40-GHz bandwidth) as well as corresponding DC power suppliers, digital multi-meter (INSTEK GDM-8245),

and measuring tools including the same apparatuses as in **Figure 4**.

**182**

**Figure 11.**

**Figure 10.**

*Common testbed for measuring static and dynamic characteristics of the VCSEL-PD under test, where ML, OM, OA, OCL, DC, DM, OSA, OPM, and RF VNA stand for master laser, optical modulator, optical amplifier, optical circulator, DC source, digital multi-meter, optical spectrum analyzer, optical power meter, and RF vector network analyzer, respectively. (optical connections are painted in red, electrical connections – in black).*

Figures below demonstrate the results of measuring the key static and dynamic characteristics of the free-running VCSEL-PD using reverse DC bias from DC3. First, **Figure 13** presents dark current vs. reverse voltage characteristic. For the device under test, dark current value is not more than 70 nA at a reverse voltage up to 3 V.

**Figure 14** presents the effect of reverse bias voltage on photocurrent of the VCSEL-PD under test at the 1560.95-nm incident optical powers of 1, 2, and 3 mW.

**Figure 15** characterizes photocurrent vs. incident optical power of 1560.95-nm wavelength for the VCSEL-PD under test at reverse voltage of 1 V. As one can see from the Figure, the current responsivity at the initial segment of the characteristic is about 0.2 A/W, and the threshold of its linearity at a level of −1 dB is approximately 2.6 mW.

**Figure 13.** *Dark current vs. reverse voltage characteristic.*

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

**Figure 14.** *Photocurrent vs. reverse voltage and incident optical powers for the free-running VCSEL-PD under test.*

**Figure 15.** *Photocurrent vs. incident optical power of the VCSEL-PD under test.*

**Figure 16** demonstrates a fine optical spectral characteristic for the VCSEL-PD under test at reverse voltage of 1 V. As one can see, due to the built-in optical cavity, the characteristic has clear resonance properties with a full bandwidth at half level of about 70 GHz.

Finally, **Figure 17** demonstrates small-signal frequency characteristic of the free-running VCSEL-PD under test at the modulating RF power of −5 dBm and the reverse voltage of 1 V. As one can see from the Figure, the total gain for EOC + OEC at the modulating frequency of 1 GHz is −38 dB at the optical power of 1 mW (brown curve 1), −28 dB at the power of 2 mW (pink curve 2), and − 23.5 dB at the power of 3 mW (blue curve 3). Moreover, for all the powers, −3-dB bandwidth is near 3.5 GHz and − 5-dB bandwidth is near 5 GHz.

**185**

**Figure 17.**

**Figure 16.**

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

The following outcomes can be drawn from this sub-section.

BPDV2150 (5–200 nA) used in the further study.

*Small-signal RF characteristic of the free-running VCSEL-PD under test.*

taken into account in the course of further research.

• The results of measuring the dark current (**Figure 13**) showed their comparability with the specifications of the inherent pin-photodetector of model

(**Figure 14**) made it possible to determine their significant influence. In this case, the photocurrent value increases with the optical power, which corresponds to theory and practice for inherent InP-based pin-photodiodes [24]. Also, the minimum value of an operating reverse voltage is 1 V, which was

• The study of the photocurrent vs. reverse voltage and optical powers

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

*Resonance features of the VCSEL-PD under test.*

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

**Figure 16.** *Resonance features of the VCSEL-PD under test.*

#### **Figure 17.**

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

**Figure 16** demonstrates a fine optical spectral characteristic for the VCSEL-PD under test at reverse voltage of 1 V. As one can see, due to the built-in optical cavity, the characteristic has clear resonance properties with a full bandwidth at half level

*Photocurrent vs. reverse voltage and incident optical powers for the free-running VCSEL-PD under test.*

Finally, **Figure 17** demonstrates small-signal frequency characteristic of the free-running VCSEL-PD under test at the modulating RF power of −5 dBm and the reverse voltage of 1 V. As one can see from the Figure, the total gain for EOC + OEC at the modulating frequency of 1 GHz is −38 dB at the optical power of 1 mW (brown curve 1), −28 dB at the power of 2 mW (pink curve 2), and − 23.5 dB at the power of 3 mW (blue curve 3). Moreover, for all the powers, −3-dB bandwidth is

near 3.5 GHz and − 5-dB bandwidth is near 5 GHz.

*Photocurrent vs. incident optical power of the VCSEL-PD under test.*

**184**

of about 70 GHz.

**Figure 15.**

**Figure 14.**

*Small-signal RF characteristic of the free-running VCSEL-PD under test.*

The following outcomes can be drawn from this sub-section.


### **3.2 Forward-biased optically injection locked mode**

In this sub-section, based on the results of the Section 2 associated with the improving single-frequency regime and broadening the modulation bandwidth under OIL, as well as on the work of other authors [18–21], we will investigate the static and dynamic characteristics of the OIL-VCSEL-PD as a separate device. The measurements will be carry out using the testbed of **Figure 12** by replacing the block "VCSEL-PD under test" with the block-diagram shown in **Figure 9**.

**Figure 18** addresses a photocurrent offset of the OIL-VCSEL-PD under test vs. incident OIL power at the free-running injection currents of 4, 5, and 6 mA, which according to voltage–current characteristic of **Figure 5(b)** corresponds to the forward DC voltages of 1.35, 1.5, and 1.67 V, respectively. As one can see from the Figure, the values of photocurrent offset depend on both the DC bias voltage that is in analogy with VCSEL-PD (see **Figure 14**) as well as on the incident optical power. In the result, the average photocurrent responsivity is varied from 0.03 A/W at +1.67 V to 0.04 A/W at +1.5 V.

**187**

**Figure 19.**

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

**Figure 19** presents photocurrent offset of the OIL-VCSEL-PD under test vs. detuning between the wavelengths of VCSEL-PD in OIL mode and VCSEL in freerunning mode (see **Figures 6** and **10**) at incident OIL power of 2 mW and forward bias voltage of 1.5 V. A sharp drop at the detuning near +0.55 nm means that the synchronization is lost. On the other hand, a sharp peak at the offset near −0.55 nm means a hop of locking from VCSEL's fundamental mode to the side mode (see

Finally, **Figure 20** presents small-signal frequency characteristic of the OIL-VCSEL-PD under test (brown curve 1) at the incident OIL power of 2 mW and detuning of +0.3 nm. Here, for comparison, a similar characteristic for an inherent pin-photodetector is shown (blue curve 2), as well as the response in the absence of

• Studying the current responsivity of the OIL-VCSEL-PD vs. the forward DC bias voltage (**Figure 18**) showed that it reaches a maximum of 0.04 A/W at a voltage of 1.5 V, which is 5 times lower compared to the free-running

• Studying the current responsivity of the OIL-VCSEL-PD vs. the detuning between the wavelengths of VCSEL-PD in OIL mode and VCSEL in free-running mode (**Figure 19**) showed that the locking range inside the fundamental mode is near 1.1 nm and the average slope of current responsivity

• Studying the small-signal frequency characteristic of the OIL-VCSEL-PD (**Figure 20**) showed that locking emission of the master laser leads to an approximately 30 dB increase in the gain, but its bandwidth is significantly lower compared to the similar characteristic of **Figure 11**. The reason for this effect is the high loss (7.2 dB) in the optical probe (see Section 2), which must

is 0.7 mA/nm, which must be taken into account in further studies.

be taken into account in the course of further studies.

*Photocurrent offset of the OIL-VCSEL-PD under test vs. detuning.*

The following outcomes can be drawn from this sub-section.

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

**Figure 6**).

an OIL signal (green curve 3).

VCSEL-PD (see **Figure 15**).

**Figure 18.** *Photocurrent offset of the OIL-VCSEL-PD under test vs. incident OIL power.*

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

**Figure 19** presents photocurrent offset of the OIL-VCSEL-PD under test vs. detuning between the wavelengths of VCSEL-PD in OIL mode and VCSEL in freerunning mode (see **Figures 6** and **10**) at incident OIL power of 2 mW and forward bias voltage of 1.5 V. A sharp drop at the detuning near +0.55 nm means that the synchronization is lost. On the other hand, a sharp peak at the offset near −0.55 nm means a hop of locking from VCSEL's fundamental mode to the side mode (see **Figure 6**).

Finally, **Figure 20** presents small-signal frequency characteristic of the OIL-VCSEL-PD under test (brown curve 1) at the incident OIL power of 2 mW and detuning of +0.3 nm. Here, for comparison, a similar characteristic for an inherent pin-photodetector is shown (blue curve 2), as well as the response in the absence of an OIL signal (green curve 3).

The following outcomes can be drawn from this sub-section.


**Figure 19.** *Photocurrent offset of the OIL-VCSEL-PD under test vs. detuning.*

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

be taken into account in further measurements.

**3.2 Forward-biased optically injection locked mode**

*Photocurrent offset of the OIL-VCSEL-PD under test vs. incident OIL power.*

rable level of linearity.

+1.67 V to 0.04 A/W at +1.5 V.

• The results of measuring the dependence of the photocurrent on the incident power (**Figure 15**) showed that the current responsivity of the VCSEL-PD under test is approximately 3 times less than that of existing pin-photodetectors operating in the microwave band. However, the VCSEL-PD has a compa-

• An important distinction of the VCSEL-PD is the resonance properties

• Measurement of the low-signal frequency response of the VCSEL-PD under study (**Figure 17**) confirms the effect of incident optical power on RF gain as predicted in **Figure 14**, which corresponds to inherent InP pin-photodiodes [24].

In this sub-section, based on the results of the Section 2 associated with the improving single-frequency regime and broadening the modulation bandwidth under OIL, as well as on the work of other authors [18–21], we will investigate the static and dynamic characteristics of the OIL-VCSEL-PD as a separate device. The measurements will be carry out using the testbed of **Figure 12** by replacing the

**Figure 18** addresses a photocurrent offset of the OIL-VCSEL-PD under test vs. incident OIL power at the free-running injection currents of 4, 5, and 6 mA, which according to voltage–current characteristic of **Figure 5(b)** corresponds to the forward DC voltages of 1.35, 1.5, and 1.67 V, respectively. As one can see from the Figure, the values of photocurrent offset depend on both the DC bias voltage that is in analogy with VCSEL-PD (see **Figure 14**) as well as on the incident optical power. In the result, the average photocurrent responsivity is varied from 0.03 A/W at

block "VCSEL-PD under test" with the block-diagram shown in **Figure 9**.

(**Figure 16**) arising from the presence of a built-in optical cavity, which must

**186**

**Figure 18.**

**Figure 20.** *Small-signal frequency characteristic of the OIL-VCSEL-PD under test.*
