**1. Introduction**

As well known, a pin-structured semiconductor photodetector (PD) is the most important active element of world-spread fiber-optics communication systems (FOCS) [1], where it performs the functions of an effective opticalelectric converter (OEC) in the receiver unit located at the far end of a telecom link. Additionally, in the last decade, this device has found wide application in the intensively developing radio electronic systems (RES) for civil and military purposes based on microwave photonics (MWP) technology [2, 3]. In these systems, the MWP circuitry also contains two more requisite units that carry out electrical-optic conversion (EOC) and processing of radio-frequency (RF) signals in the optical domain [4]. Therein, the EOC operation is realized either by direct

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

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

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

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 communication network [17].

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

**177**

**Figure 1.**

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

of this approach compared to our one is the possibility of full-duplex operation in a combined EOC/OEC without switching the polarity of the power supply. In the cited papers, an integrated consideration is used that addresses the effect of expanding the bandwidth of an OIL VCSEL-based photodetector (OIL-VCSEL-PD) and experimental study of the signal quality (bit error rate) in a FOCS, when digital signals at a speed of 12 Gbps are transmitted. However, the key parameters of the OIL-VCSEL-PD itself, such as responsivity, the level of introduced noise, etc., have

As a result, in present scientific publications there is no clear answer to the question of selecting the optimal photodetector for the application in microwave- photonics circuits. Eliminating it, this Chapter addresses a comparative experimental study reviewing the known, updated, and newer results to pursue advanced performances corresponding to the key OEC parameters of an inherent pin-photodetector as well as the two VCSEL-based photodetectors in free-running or OIL mode and their applications in the last generation of key MWP circuits for prospective photonics and radio-electronics systems. In particular, pursuing the goal to discovery optimal operation regimes for the VCSEL-based RCE photodetector, Section 2 reviews key features of the C-band (1530–1565 nm) free-running LW-VCSEL as the object for investigation. In addition, Section 3 presents the updated and newer results of measuring static and dynamic characteristics for the LW-VCSEL-based RCE photodetector under study in the above-mentioned operating modes. The results of design and comparative experimental research using a typical MWP circuit that includes OEC based on an inherent pin-photodetector or two versions of VCSEL-based RCE photodetectors under study are demonstrated in

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

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

Section 4. Finally, Section 5 concludes the Chapter.

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

**2. The object for investigation**

not been studied at all.

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

of this approach compared to our one is the possibility of full-duplex operation in a combined EOC/OEC without switching the polarity of the power supply. In the cited papers, an integrated consideration is used that addresses the effect of expanding the bandwidth of an OIL VCSEL-based photodetector (OIL-VCSEL-PD) and experimental study of the signal quality (bit error rate) in a FOCS, when digital signals at a speed of 12 Gbps are transmitted. However, the key parameters of the OIL-VCSEL-PD itself, such as responsivity, the level of introduced noise, etc., have not been studied at all.

As a result, in present scientific publications there is no clear answer to the question of selecting the optimal photodetector for the application in microwave- photonics circuits. Eliminating it, this Chapter addresses a comparative experimental study reviewing the known, updated, and newer results to pursue advanced performances corresponding to the key OEC parameters of an inherent pin-photodetector as well as the two VCSEL-based photodetectors in free-running or OIL mode and their applications in the last generation of key MWP circuits for prospective photonics and radio-electronics systems. In particular, pursuing the goal to discovery optimal operation regimes for the VCSEL-based RCE photodetector, Section 2 reviews key features of the C-band (1530–1565 nm) free-running LW-VCSEL as the object for investigation. In addition, Section 3 presents the updated and newer results of measuring static and dynamic characteristics for the LW-VCSEL-based RCE photodetector under study in the above-mentioned operating modes. The results of design and comparative experimental research using a typical MWP circuit that includes OEC based on an inherent pin-photodetector or two versions of VCSEL-based RCE photodetectors under study are demonstrated in Section 4. Finally, Section 5 concludes the Chapter.
