## 1 Introduction  
Since their introduction several decades ago, ultrashort laser pulses have allowed researchers to make the most temporally precise and accurate measurements ever made. However, such measurements depend heavily on the ability to measure the pulses involved, and pulse-measurement techniques have traditionally lagged well behind pulse-generation techniques. This is because the measurement of an event has generally required the use of a shorter one, and ultrashort laser pulses are the shortest events ever generated. Even a self-referenced technique, that is, one that uses the event to measure itself--the best that could hope to be achieved--appeared insufficient, as the pulse is only as short as itself, not shorter. Twenty years ago, the single-pulse measurement problem was solved by the introduction of frequency-resolved optical gating (FROG), a self-referenced technique that completely measures the intensity and phase of one arbitrary ultrashort laser pulse [1; 2; 3]. FROG and its simplified version, GRENOUILLE [4; 5; 6], are widely used as self-referenced pulse measurement techniques today. In addition to the FROG family, other techniques, including multiphoton intrapulse interference phase scan (MIIPS) [7; 8] and spectral-phase interferometry for direct electric-field reconstruction (SPIDER) [9; 10], have also achieved common use.  
In modern ultrafast optical experiments--especially high-intensity ones where nonlinear-optical processes easily generate additional pulses at new wavelengths--there is often a need to measure two independent unknown pulses simultaneously. For example, ultrafast pump-probe spectroscopy in various applications, such as material characterization and coherent control, often uses two pulses of different color and, potentially, pulse lengths, bandwidths and complexities. Its operation or temporal-resolution optimization requires measuring both the pump and probe pulses [11; 12; 13; 14]. Also, applications involving nonlinear-optical processes, such as harmonic generation and continuum generation in optical fibers [15; 16; 17], which generate output pulses at different wavelengths, require complete information on both the input and output pulses to understand the underlying physics or to verify that the process has occurred properly. Therefore, a self-referenced technique that can measure two different, independent pulses simultaneously would be very useful. The technique should also have minimal restrictions on wavelengths, bandwidths and complexities. Single-shot operation would be even better, as pulses from amplified system with significant shot-to-shot fluctuations are involved.  
While it is always possible to measure two pulses separately using two independent measurement devices, a single device that can measure two pulses simultaneously with minimum cost, space and complexity is desirable. Over the years, researchers have proposed different techniques to accomplish this two-pulse measurement task in a single device, such as Blind-FROG [18; 19; 20], complete reconstruction of attosecond bursts (CRAB) [21; 22] very advanced method for phase and intensity retrieval of e-fields (VAMPIRE) [23; 24] and Double Blind FROG [25; 26]. Several articles have reviewed the commonly used single-pulse measurement techniques [27; 28; 29; 30], so we will focus on the two-pulse measurement problem and review the aforementioned techniques.