**1. Introduction**

Polymers have the potential to be useful for many passive and active sub-components. One benefit of polymers over other types of materials is that their physical and optical characteristics may be greatly customized by adjusting the composition and level of polymerization. By including the proper molecular moieties in the polymer chain or as side pendants, functionality may be introduced [1]. Different techniques may be used to process polymers, such as solution and gas-phase deposition, and they can be made compatible with substrate chemistry by appropriate surface functionalization (including inorganic building blocks). Large-scale, inexpensive production of polymers is also a possibility [2]. The optical waveguide (WG) is one of the fundamental components of integrated photonics [3–6]. Polymer WGs can operate in either single-mode (with core diameters between 2 μm and 5 μm) or multimode (with core dimensions generally between 30 μm and 500 μm) regimes. They are both entirely consistent with the matching optical fiber type due to the similar mode field diameter. Numerous methods, including photolithography [7], flexographic and inkjet printing [8], nanoimprint lithography [9,10], femtosecond laser processing [11], and hot embossing [12], can be used to create these WGs. A stamp structure is transferred from a stamp onto a substrate through the hot embossing process, which is a replication process. The method is appropriate for replicating structures with a millimeter to nanoscale size. Due to its potential for mass production at low cost and integration with roll-to-roll processes, hot embossing is a desirable manufacturing method for optical applications [13]. By using sustainable and efficient hybrid lithography, a three-dimensional polymer WG

<sup>1</sup> Samara National Research University, 443086 Samara, Russia

with a taper structure was exhibited and created [14]. A polymer WG and a polymer taper structure were created using grayscale lithography and hybrid lithography, respectively. Gray-scale lithography was intended for laser ablation and shadow aluminum evaporation. The laser strength, the rate of ablation, and the thickness of the aluminum may all be adjusted to alter the length of the grayscale zone, which ranges from 20 to 400 μm [14].

Due to their flexible processibility and integration over inorganic counterparts, polymer optical WG devices are crucial in several rapidly evolving broadband communications domains, including optical networking, metropolitan/access communications, and computer systems [15]. Owing to their many benefits, they are also a perfect integration platform for the insertion of foreign material systems like YIG (yttrium iron garnet) and lithium niobate in addition to semiconductor devices like lasers, detectors, amplifiers, and logic circuits into etched grooves in planar lightwave circuits to enable full amplifier modules or optical add/drop multiplexers on a single substrate. Additionally, optical polymers may be vertically combined to produce 3D and even all-polymer integrated optics because of their flexibility and durability combination [14].

Polymer WG-based optical sensors can be competitors to devices based on photonic integrated circuits (PICs). Such devices are mainly manufactured based on MPW (multiproject wafer) on SOI, SiN platforms, or based on Group III–V semiconductors, primarily InP or GaAs. Type III–V platforms can provide a wide range of active devices, but they are poorly applicable to passive elements due to high attenuation and low contrast [16–18]. Figure 1 shows the emission wavelength coverage of semiconductor lasers based on III–V platforms [19]. One can see that their emission region is lower than the Si transparency window. The transparency range of a polymer WGs depends on the specific polymer material being used. In general, the transparency range is in the near-infrared (NIR) region of the electromagnetic spectrum, usually from 700 nm to 1700 nm. Therefore, one can easily combine them with III–V group light sources in the sensing system's design. However, some polymers have higher transparency windows extending into the visible region or even beyond the short-wave infrared (SWIR) region [20,21]. It is also worth noting that the transparency range of polymer WGs can be affected by various factors such as processing conditions, doping, and absorption or scattering losses. SOI platforms, which are wellcompatible with traditional CMOS electronics, have high contrast and small allowable bend radii [22], making it possible to design small-sized passive sensor devices but not active components. For example, a sensor based on a Mach–Zehnder interferometer (MZI) with a double-slot hybrid plasmonic WG [23] provides a high sensitivity of up to 1061 nm/RIU in liquid refractometry. The micro-ring resonator-based sensors' sensitivity on the SOI platform is less than 100 nm/RIU. However, the application of a subwavelength grating micro-ring makes it possible to achieve a sensitivity of 672.8 nm/RIU [24,25]. Hybrid integration significantly expands the capabilities of the SOI and III–V platforms, allowing the design of complex sensing systems, including active devices [26,27]. It is also possible to expand the capabilities of the SOI platform using IMOS (indium phosphide membrane on silicon) technology [28].

The paper is organized in the following manner. Section 2 provides a piece of information on the characteristics of polymer WGs. There are several polymer materials commercially available and being used in research for the development of photonic devices as discussed in Section 3. The extraordinary optical properties of these polymer materials include low optical losses at operating wavelengths, well-controlled and tunable refractive indices, resistance to temperature and chemicals, mechanical stability in a variety of environments, and environmentally friendly fabrication techniques. Afterward in Section 4, polymer WG-based sensors are discussed. We have dedicated our research to polymer WG-based biosensors (Section 4.1), gas sensors (Section 4.2), temperature sensors (Section 4.3), and mechanical sensors (Section 4.4), which are at present the main focal point of investigation. The paper finishes with a brief conclusion and outlook as presented in Section 5. The applications presented in this paper are shown in Figure 2.

**Figure 1.** Transparent window of polymers, silicon, and silicon dioxide, and emission wavelength coverage of semiconductor lasers based on different III–V active regions. InP-based type-I, type-II, and GaSb-based type-I quantum well (QW) diode lasers, GaSb-based interband cascade lasers (ICLs), and quantum cascade lasers (QCLs) are included [19].

**Figure 2.** Polymer WG-based sensors employed in (**a**) biosensing [29], (**b**) gas sensing [30], (**c**) temperature sensing [31], and (**d**) mechanical sensing applications [32], are discussed in this review.
