**2. The object for investigation**

For today, several technologies yielding LW-VCSEL with acceptable performances have been developed. Among them, wafer-fused LW-VCSELs under research (**Figure 1**) employing strained InP/InAlGaAs quantum well (QWs) active region, tunnel junction (TJ) for carrier and optical confinement, and distributed Bragg reflectors (DBR), have also reached the industrial production stage and proven reliability [8]. A particular preference of these LW-VCSELs is in covering the full ITU-T spectral range from O-band to U-band. Concerning MWP approach, an outstanding feature of LW-VCSELs is their compatibility with future large-scale silicon-based heterogeneous photonics integrated circuits [22], which should

#### **Figure 1.**

*Schematic of wafer-fused LW-VCSEL from beam express LLC, Switzerland.*

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

reducing the cost of the device by combining EOC and OEC functions.

latter was controlled by adjusting the diameter of the active layer.

FOCSs [7] as well as in MWP circuits [8].

modulation of a semiconductor laser's injection current or by external modulation of an optical modulator pumped by a laser source power. After optical processing such as transmission, amplifying, filtering, time delay, etc., an OEC operation must be performed, for the implementation of which a pin-photodetector is used. Thus, unlike traditional FOCSs, both a laser and a photodetector must be present in a common MWP circuit, which paves the way for its simplifying and, therefore,

The study of the structures and constructions of modern photodiode and widespread edge emitting laser showed that the former cannot operate in the lasing mode, since there is no amplification layer and optical resonators that provide positive feedback. In addition, the latter is not suitable for operation in a photodetector mode, in principle, since it has a completely different design and even a very small reverse biasing leads to its failure. However, there is a semiconductor laser of a different design: a vertical cavity surface-emitting laser (VCSEL) with an epitaxial structure similar to a photodetector [5, 6], and its long-wavelength version (LW-VCSEL) has a great potential for the application in modern and prospective

In general, a resonant cavity enhanced (RCE) PD based on a pin-photodiode or a Schottky-barrier photodiode is a long-time known optoelectronic device that overcomes the fundamental drawback of an inherent photodetector associated with a compromise between bandwidth and sensitivity [9, 10]. Its design with an active depletion region between two multilayer mirrors of a Fabry-Perot resonator is similar to a VCSEL. Modern development follows the path of researching and fabricating both individual RCE-PD chips in the short-wavelength or long-wavelength telecom spectral range [11, 12], as well as monolithically integrated chips containing on one substrate an optoelectronic pair based on a VCSEL and a RCE-PD [13]. So that, during a literature search, a publication was found [14] reporting the results of an experimental study of RCE photodiode based on a short-wavelength VCSEL with a quantum-well active region operating in the photovoltaic mode or in reverse bias mode. This technological study was carried out in order to determine the conditions for ensuring the maximum quantum efficiency of the OEC, which was regulated by sequentially etching the layers of the upper mirror, as well as the conditions for ensuring the maximum width of the frequency characteristic at the output. The

The motivation for our recent investigation in this direction [15] was to measure the static and dynamic characteristics of the particular LW-VCSEL sample without any structural changes. For this goal, we simply reversed the DC bias polarity to assess the efficiency of its use as part of an optoelectronic coupler based on two identical LW-VCSEL chips, one of which worked as a laser source, and the other as a photodetector. An additional goal was to demonstrate the effective operation of a LW-VCSEL-based RCE photodetector (VCSEL-PD) in an economical photoreceiver for the currently widespread digital FOCS with dense wavelength division multiplexing (DWDM), due to the absence of a spectral demultiplexer needed for a standard DWDM FOCS. Leveraging this concept, two other applications combining LW-VCSEL in the laser and photodetector modes have recently been investigated including the uses in a high-speed optoelectronic switch device for integrated photonics-based optical beamforming network [16] and in cost-efficient optoelectronic frequency-converting transceiver for a base station of 5G cellular

To date, there is one more version of LW-VCSEL-based photodetector [18, 19], where a new concept using such an effective laser technique, especially for lowpower VCSEL, as optical injection locking (OIL) [20, 21], was proposed and preliminary investigated. From the operational point of view, an obvious advantage

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communication network [17].

provide many advantages when implementing MWP devices and techniques. The processing of the double-fused VCSEL wafer includes reactive ion etching in top DBR, selective chemical etching in the InAlGaAs/InP active cavity region, dielectric deposition, dry etching, and e-beam deposition of metals for contacts [8]. The much smaller size of the active region (D) in comparison with the edge emitting laser requires several times higher reflection coefficients of the upper and lower mirrors to generate lasing conditions, which is provided due to high-Q DBRs based on 30 or more intermittent layers of GaAs/AlGaAs. These mirrors are fused to top and bottom parts of an each active region before dicing [23].

**Figure 2** shows a microscope view of a tiny piece of a laser wafer with formed electrical pads. As seen, the processed VCSEL wafer offers on-wafer testing possibility that decreases manufacturing cost as compared to edge emitting lasers. Moreover, a pattern of the full wafer-electroplated anode and cathode pads is designed in such a way to ensure the correct testing using a standard microwave probe of GS-type.

All further measurements will carry out on the Probe Station (PS) EP6 from Cascade Microtech. The workbench including thermo-electrical cooler for temperature controlling, coplanar RF probe (on the left), and fiber-optics probe (on the right) is shown in **Figure 3**.

**Figure 2.**

*A microscope view of the VCSEL wafer under study.*

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**Figure 5.**

*complete output power [8].*

**Figure 4.**

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

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

and needed for MWP characterization dynamic characteristics of the LW-VCSEL under test in free-running mode of its operation [8, 23] that will use further for examination of the same device in RCE photodetector modes. The testbed (see **Figure 4**) operates in three ranges: direct current (DC), RF, and C-band (1530– 1565 nm) optical ones. Besides the VCSEL under test, it contains a set of accessories such as optical coupler (Opneti, CP10/90–1550), reference photodiode (Finisar, BPDV2150, 43-GHz bandwidth, 0.6-A/W responsivity), bias-T (Pasternack, PE1BT-1002, 40-GHz bandwidth) as well as corresponding DC power suppliers and measuring tools including optical spectrum analyzer (Yenista OSA20), optical power

meter (EXFO PM-1100), RF vector network analyzer (Agilent E8363B).

chip when registering its complete output power [8].

**Figure 5(a)** depicts a PS-assisted light-current characteristic (LCC) of the LW-VCSEL under test emitting in the fundamental wavelength of 1560.95 nm at the room temperature. For the device under test, the threshold current is near 2.3 mA and a quasi-linear dependence of coupled optical power vs. current was observed up to 9 mA. To estimate the losses introduced by the optical probe, **Figure 5(b)** shows typical LCCs and a voltage–current characteristic of a C-band LW-VCSEL

*Common testbed for measuring static and dynamic characteristics of the object for investigation, where OC, RPD, OSA, OPM, and RF VNA stand for optical coupler, reference photodiode, optical spectrum analyzer, optical power meter, and RF vector network analyzer, respectively. (optical connections are painted* 

*Light-current characteristics of the VCSEL chip under study: (a) Probe station-assisted; (b) Registering its* 

Below we, using widespread techniques and procedures, will present typical static

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

**Figure 3.** *The workbench for VCSEL chip's study.*

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

Below we, using widespread techniques and procedures, will present typical static and needed for MWP characterization dynamic characteristics of the LW-VCSEL under test in free-running mode of its operation [8, 23] that will use further for examination of the same device in RCE photodetector modes. The testbed (see **Figure 4**) operates in three ranges: direct current (DC), RF, and C-band (1530– 1565 nm) optical ones. Besides the VCSEL under test, it contains a set of accessories such as optical coupler (Opneti, CP10/90–1550), reference photodiode (Finisar, BPDV2150, 43-GHz bandwidth, 0.6-A/W responsivity), bias-T (Pasternack, PE1BT-1002, 40-GHz bandwidth) as well as corresponding DC power suppliers and measuring tools including optical spectrum analyzer (Yenista OSA20), optical power meter (EXFO PM-1100), RF vector network analyzer (Agilent E8363B).

**Figure 5(a)** depicts a PS-assisted light-current characteristic (LCC) of the LW-VCSEL under test emitting in the fundamental wavelength of 1560.95 nm at the room temperature. For the device under test, the threshold current is near 2.3 mA and a quasi-linear dependence of coupled optical power vs. current was observed up to 9 mA. To estimate the losses introduced by the optical probe, **Figure 5(b)** shows typical LCCs and a voltage–current characteristic of a C-band LW-VCSEL chip when registering its complete output power [8].

#### **Figure 4.**

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

and bottom parts of an each active region before dicing [23].

probe of GS-type.

right) is shown in **Figure 3**.

provide many advantages when implementing MWP devices and techniques. The processing of the double-fused VCSEL wafer includes reactive ion etching in top DBR, selective chemical etching in the InAlGaAs/InP active cavity region, dielectric deposition, dry etching, and e-beam deposition of metals for contacts [8]. The much smaller size of the active region (D) in comparison with the edge emitting laser requires several times higher reflection coefficients of the upper and lower mirrors to generate lasing conditions, which is provided due to high-Q DBRs based on 30 or more intermittent layers of GaAs/AlGaAs. These mirrors are fused to top

**Figure 2** shows a microscope view of a tiny piece of a laser wafer with formed electrical pads. As seen, the processed VCSEL wafer offers on-wafer testing possibility that decreases manufacturing cost as compared to edge emitting lasers. Moreover, a pattern of the full wafer-electroplated anode and cathode pads is designed in such a way to ensure the correct testing using a standard microwave

All further measurements will carry out on the Probe Station (PS) EP6 from Cascade Microtech. The workbench including thermo-electrical cooler for temperature controlling, coplanar RF probe (on the left), and fiber-optics probe (on the

**178**

**Figure 3.**

**Figure 2.**

*The workbench for VCSEL chip's study.*

*A microscope view of the VCSEL wafer under study.*

*Common testbed for measuring static and dynamic characteristics of the object for investigation, where OC, RPD, OSA, OPM, and RF VNA stand for optical coupler, reference photodiode, optical spectrum analyzer, optical power meter, and RF vector network analyzer, respectively. (optical connections are painted in red, electrical connections – in black).*

#### **Figure 5.**

*Light-current characteristics of the VCSEL chip under study: (a) Probe station-assisted; (b) Registering its complete output power [8].*

**Figure 6** shows an example of a PS-assisted spectral characteristic of the LW-VCSEL under test at the current of 6 mA, where a fundamental mode with the coupled power of −10 dBm at the wavelength of 1560.95 nm and a side-mode suppression ratio of 52 dB are observed.

In addition, **Figure 7** presents the spectral evolution of VCSEL emission with currents and temperature.

**Figure 8** presents the small-signal transmission gain (TG) of an optoelectronic pair comprising the LW-VCSEL under test and RPD (see **Figure 4**). As one can see, at lower modulation frequencies the TG value is −30 dB and the -3 dB bandwidth of the LW-VCSEL under test is 3.7 GHz at 3-mA and more than 9 GHz at 10-mA bias current.

For the sake of completeness, we will end this section with a couple of examples characterizing the VCSEL under study in an optically injection locked mode.

**Figure 6.** *Example of a spectral characteristic of the LW-VCSEL under test.*

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**Figure 9.**

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

The measurements will be made using the testbed of **Figure 4** by replacing the

**Figures 10** and **11** exemplify an optical spectrum and small-signal modulation

The following outcomes for the further study can be drawn from this section.

• As follows from a comparison of the spectral characteristics for VCSEL in freerunning (**Figure 6**) and OIL (**Figure 10**) modes, the effect of optical injection locking leads to a more than 10-dB increase in emitted power and to a pure singlefrequency spectrum with almost 10-dB gain in side-mode suppression ratio.

• As it can be observed from **Figure 7**, combinations of temperature and driving current allow tuning the emission wavelength inside 12 nm with the separate current tuning of near 0.25 nm/mA and temperature tuning of near 0.1 nm/°C.

• As follows from a comparison of the small-signal modulation characteristics for VCSEL in free-running mode (**Figure 8**) at current of 8 mA and in OIL mode (**Figure 11**), effect of optical injection locking leads to more than 4-fold

*Conceptual block-diagram of OIL-VCSEL, where ML, OA, and OCL stand for master laser, optical amplifier, and optical circulator, respectively. (optical connections are painted in red, electrical connections – in black).*

• Comparison of LCC (**Figure 5(a)**) with the previously obtained measurement results for a similar characteristic of a VCSEL chip when registering a complete output power (see **Figure 5(b)**) allows us to assess the probe-assisted coupled factor at a level of 19% (losses of about 7.2 dB), which will be taken into

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

*Small-signal transmission gain characteristics of the wafer-fused LW-VCSEL under test.*

characteristics of tested VCSEL in OIL mode.

**Figure 8.**

account in further estimates.

modulation bandwidth.

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

**Figure 7.** *Spectral evolution of the tested VCSEL emission with current and temperature.*

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

**Figure 8.** *Small-signal transmission gain characteristics of the wafer-fused LW-VCSEL under test.*

The measurements will be made using the testbed of **Figure 4** by replacing the block "VCSEL under test" with the block-diagram shown in **Figure 9**.

**Figures 10** and **11** exemplify an optical spectrum and small-signal modulation characteristics of tested VCSEL in OIL mode.

The following outcomes for the further study can be drawn from this section.


#### **Figure 9.**

*Conceptual block-diagram of OIL-VCSEL, where ML, OA, and OCL stand for master laser, optical amplifier, and optical circulator, respectively. (optical connections are painted in red, electrical connections – in black).*

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

suppression ratio of 52 dB are observed.

currents and temperature.

current.

**Figure 6** shows an example of a PS-assisted spectral characteristic of the LW-VCSEL under test at the current of 6 mA, where a fundamental mode with the coupled power of −10 dBm at the wavelength of 1560.95 nm and a side-mode

In addition, **Figure 7** presents the spectral evolution of VCSEL emission with

**Figure 8** presents the small-signal transmission gain (TG) of an optoelectronic pair comprising the LW-VCSEL under test and RPD (see **Figure 4**). As one can see, at lower modulation frequencies the TG value is −30 dB and the -3 dB bandwidth of the LW-VCSEL under test is 3.7 GHz at 3-mA and more than 9 GHz at 10-mA bias

For the sake of completeness, we will end this section with a couple of examples characterizing the VCSEL under study in an optically injection locked mode.

**180**

**Figure 7.**

**Figure 6.**

*Spectral evolution of the tested VCSEL emission with current and temperature.*

*Example of a spectral characteristic of the LW-VCSEL under test.*

#### **Figure 10.**

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

#### **Figure 11.**

*Small-signal modulation characteristics of OIL-VCSEL under test in free-running mode (1), and in OIL mode when the power of the ML is 5 dBm (2) or 8 dBm (3).*
