###### Abstract  
In this article, we report on an optical real-time autocorrelator readout with a 5 Hz refresh rate, equipped with a transimpedance amplified photodetector based on the two-photon absorption (TPA) of semiconductor photodiodes (PDs) for ultrashort (1 \(<\) ps) pulse measurement. By replacing the GaP PD of a commercial TPA detector with GaAsP and Si PD elements, we demonstrated that the spectral response based on the TPA of each photodetector followed the linear response of the corresponding semiconductor PD within accessible wavelength regions. The TPA spectral response of the GaAsP detector exhibited a peak at 1200 nm and a long wavelength limit near 1300 nm. The TPA spectral response of the Si detector exhibited a short wavelength limit near 1170 nm and a linear response up to 1300 nm. The two types of PD were compared with the characteristics of the GaP photodiode. These photoconductive detectors are efficient, compact, and robust sensors and can be used to diagnose the pulse characteristics of ultrafast fiber lasers and light sources near IR wavelengths. real-time autocorrelator; two-photon absorption; photoconductive detector; GaP; Si; GaAsP; photodiode +
Footnote †: journal: Applied Sciences  
and GaAs PMTs [17; 18], in standard autocorrelators. Multiple quantum-well waveguides and microcavity-structured optoelectronic devices have also been demonstrated [19; 20]. Semiconductor sensors with linear absorption responses are preferred, as they offer high versatility, ease of integration with compact devices, and insensitivity to phase-matching and polarization restrictions for pulse diagnostics. Over the last two decades, the field of pulse diagnostics has progressed rapidly, together with a multitude of ultrafast and ultrahigh laser developments. Spectrograms of frequency-resolved optical gating (FROG) [21] and spectral shearing sonograms for direct electric-field reconstruction (SPIDER) [22] have been developed to realize ultrafast oscilloscopes that display the temporal and spectral profiles of input pulses (intensity and phase) at rates of several Hertz. However, such techniques require extended setups--such as mini-spectrometers or cameras--and proprietary retrieval algorithms. These techniques still require nonlinear crystals or media to obtain spectrograms or sonograms for retrieving the amplitude and phase of ultrashort pulses. Although IAC does not provide a direct measure of the phase structure of an input pulse, it nonetheless provides useful information about coherent artifacts and the presence of coherent sub-structures, and provides a clear measure of second-order coherence characteristics for determining the pulse characteristics [23]. Stable ultrafast laser systems can be challenging to develop and maintain; this is because multiple scatter pulses with variable separations and relative phases with respect to the main pulses can be easily induced by over-pumping the power and due to the improper choice of output coupler transmittance. Moreover, modern FROG- and SPIDER-based techniques are unable to detect or retrieve unstable satellite pulses in all cases [24]. Consequently, traditional pulse autocorrelation measurements, regardless of sum-frequency wave mixing, are preferred in simple ultrafast oscilloscopes to monitor stable single-pulse trains. Recently, the TPA effect in commercial complementary metal-oxide-semiconductor (CMOS) webcams has been applied to develop a cost-effective NIR spectrometer capable of generating nonlinear spectral intensity and its modulated spectrum [25]. Most photocurrents induced by the photovoltaic TPA effect on various semiconductors are as low as sub-nano-ampere-scale, so they need to be amplified using external current amplifiers to convert the current into a voltage [26].