## 4 Conclusions  
A real-time readout autocorrelator with a 5 Hz refresh rate, based on the TPA effect, inside a commercial GaP and Si detector, as well as an alternative to a GaAsP PD, embedded with a transimpedance amplifier for switchable gain, was demonstrated. We investigated the TPA spectral responses of GaP, GaAsP, and Si photoconductive detectors by analyzing the amplitude and pulse width of an AC function, collected and fitted from TPA-induced intensity AC traces across a spectral wavelength range of 680-1300 nm. The TPA spectral response for the GaP detector exhibited a peak at 860 nm and a long wavelength limit near 970 nm, faithfully following the linear response curve across a 680-970 nm wavelength range. The TPA spectral response of the GaAsP detector exhibited a peak at 1200 nm and a long wavelength limit near 1300 nm, accurately following the linear response curve across the 685-1300 nm wavelength region. The TPA spectral response of the Si detector exhibited a peak at 1200 nm and a long wavelength limit near 1300 nm. The Si detector showed that the TPA spectral response resembled the linear response curve, based on the results obtained within the narrow 1170-1300 nm wavelength region. pulse width measurement experiment techniques using photodiodes are a long-standing research topic [1; 2]. For the first time, 6 fs ultrafast pulse diagnosis was possible from a photodiode using GaAsP [1]. Subsequently, the development of an autocorrelation device for pulse width diagnosis usinggroup 3-5 [4; 5; 6; 7], group 2-6 [11], and group 4 compound Si semiconductor [3; 9; 10; 12] and an MQW/a waveguide [15] was carried out consistently. These methods were performed as photovoltaic methods that detect signals without applying a reverse voltage across the photodiode. For this reason, the pulse width was measured using a sophisticatedly designed current amplifier to amplify very small currents generated from the photodiode. In this study, a transimpedance amplification method was used that effectively converts the minute current excited by the TPA phenomenon into a voltage signal by applying reverse voltage within the breakdown voltage range to both ends of the photodiode. That is, by driving in photoconductive mode, it was possible to acquire a large signal without using a separate current amplifier and an additional circuit for noise removal. Currently, photoconductive detectors equipped with GaP and GaAsP PDs have been discontinued commercially and can be difficult to obtain, and are available online only in small quantities. However, when high-power ultrafast pulse lasers are used for only a short time, the cumulative effect caused by heat is small, so they will be used for sterilization, treatment, and diagnosis, such as processing and cutting using melting and ablation, or medical and bio-imaging fields. For this, the real-time measurement of pulses is essential. In particular, GaAsP photodetectors are remarkably compact and robust sensors that can be used to diagnose pulse characteristics for high-power ultrafast fiber lasers and measurement systems using Yb-doped solid-state lasers near 1.0-1.1 \(\upmu\)m, while Si photodetectors are promising for use in the optical communication field using Er-doped fiber lasers and femtosecond optical parametric amplifiers near 1.5-2.0 \(\upmu\)m.  
Finally, we recognize the need to lower or eliminate the high DC offset-voltage induced by linear absorption because of PD imperfections that limit the dynamic range of autocorrelation function, as found in some of the commercial photoconductive detectors mentioned above. Moreover, new photoconductive detectors--such as GaN and AlGaN detectors for ultrashort pulse diagnoses in visible and/or ultraviolet wavelength ranges--could be developed to increase the versatility and convenience of conventional real-time autocorrelators as ultrafast oscilloscopes. H.-S.K. : Methodology; Setup Arrangement; Analysis; Validation; Writing--Review and Editing. Y.-S.L.