# All-Optical Method of Determining Laser Power from the Photomechanical Effect
## 1 Introduction  
Optical power is typically measured by transducing a beam's light energy into an electrical signal. The pyroelectric effect is the most notable mechanism for converting light energy from pulsed lasers into an electric signal [1; 2; 3]. A short pulse of light absorbed at the surface of a sensor is converted to thermal energy which in turn causes a temporary shift in the voltage across a pyroelectric crystal. The high sensitivity pyroelectric sensors to transient signals is critical to their operation, where they are designed to measure the energy of the laser pulse. The most common class of CW laser power sensor absorbs light at its surface which produces heat. If the other side of the sensor is held at a constant lower temperature, a gradient forms and heat flows to the lower temperature side. The thermal power is then converted to an electrical signal by either thermocouples or semiconductors with large thermoelectric efficiencies [4; 5; 6; 7]. Thermoelectric and thermopile laser sensors measure an assumed static temperature gradient from a CW laser source and ill-equipped to measure sub-microsecond heat impulses. Likewise, pyroelectric sensors that measure transient signals are ill-suited for CW laser power measurements. Another class of laser power sensor directly converts light energy into an electrical current via exciton formation and separation near a photodiode's \(pn\)-junction [8; 9]. The photodiode sensor is typically used for CW laser power measurements; however, some low-energy laser pulse sensors are based on photodiodes (for example, a broad assortment of commercial sensors are available from Ophir ([[https://www.ophiropt.com](https://www.ophiropt.com)]([https://www.ophiropt.com](https://www.ophiropt.com)), accessed 12 October 2022) or Gentec-EO ([[https://www.gentec-eo.com/](https://www.gentec-eo.com/)]([https://www.gentec-eo.com/](https://www.gentec-eo.com/)), accessed 12 October 2022)). The operating wavelength range of laser thermal power/energy sensors depends on the spectral profile of the light-absorbing material. Thus, the surface of a laser thermal sensor can be coated by a thin film to tune the sensor's spectral response. The wavelength range of photodiode sensors is limited to response of the diode material [10]. For example, some Si photodiodes protected by fused silica windows have a response range from about200 nm to 1100 nm [11], whereas InGaAs photodiodes have a near infrared response range typically between 800 nm and 1800 nm [12]. In this paper, we introduce a new type of CW laser power sensor based on the macroscopic deformation of a material under illumination, which is referred to as the photomechanical effect [13, 14]. There are many types of materials that can have large photomechanical strains over a broad range of wavelengths making them well-suited for laser power measurements. A large assortment of photomechanical materials can be synthesized, fabricated, or purchased at relatively low cost. Some of these materials can be processed with ease using small benchtop equipment. There are a few well-studied mechanisms responsible for the photomechanical effect. Light striking a piezoelectric material can cause a photostive strain from the photovoltaic effect [15, 16]. Another mechanism that causes light-induced deformations is observed in prestrained polymers/elastomers crosslinked with nematic mesogens [17, 18] and doped with azobenzene and other photochromic dyes [19, 20, 21, 22, 23]. Trans-cis photoisomerization of the photochromic molecules disrupts the orientational order of the mesogens which leads to a contraction along the director in the nematic phase [24, 25, 26, 27, 28]. Photomechanical effects are also observed in photochromic polymers, which are polymeric materials with backbones that contain photochromic moieties, where trans-cis photoisomerization can cause macroscopic mechanical deformations [29, 30, 31, 32, 33, 34, 35, 36].