This application kit addresses a means for monitoring the spectral profile of a laser pulse as a function of time. This technique is known as frequency-resolved optical gating (FROG) which is a general technique that can be employed in a variety of experimental geometries. The focus of this kit is on two geometries: self-diffraction (SD FROG) for amplified systems and second harmonic (SHG FROG) for oscillators. FRG-KT will work for wavelengths between 480 and 2000 nm depending on the nonlinear crystal used.
As a consequence of the time-bandwidth uncertainty principle, ultrashort laser pulses carry significant bandwidth. If the spectral components that fall under the bandwidth of the laser pulse are time coincident (figure 1.1), the pulse is said to be at its transform limit. A transform-limited pulse implies that the pulse duration is minimized.
Figure 1.1: The temporal relationship between selected Fourier components of a transform-limited pulse.
Material properties such as dispersion alter the (phase) relationship between the spectral components of light, effectively separating the blue and red components of the pulse in time (figure 1.2). This effect is known as chirp, and for an ultrashort pulse, serves to elongate the pulse in time. Although autocorrelation (see Application Note 27 - Long Scan Autocorrelator (www.newport.com/AppsNote27) can measure the duration of an ultrashort pulse, it is unable to characterize the phase relationship between different spectral components. The primary reason for this is that autocorrelators utilizes a single element photodetector that effectively integrates over the spectral profile of the pulse.
Figure 1.2: The temporal relationship between selected Fourier components of a positively chirped pulse.
Fortunately, techniques exist that monitor the spectral profile of a pulse as a function of time. These techniques allow for the complete reconstruction of the electric field. Of these techniques, frequency-resolved optical gating (FROG) is arguably the most straightforward and easiest to implement. As mentioned earlier, FROG allows for full phase retrieval of the input field without the ambiguity associated with autocorrelation. Application Note 33 presents a simple, low dispersion, easy-to-use, general purpose FROG device that can readily be implemented on the Newport Long Scan Autocorrelator platform as described in Application Note 27.
FROG is a general technique and can be employed in a variety of experimental geometries, a few of which are listed in Table 1.i
|
FROG Geometry |
Works with Oscillator |
Works with Amplifier |
Characterize Sign of Chirp |
Relative Complexity |
SH FROG
(Second Harmonic) |
Yes |
Yes |
No |
Low |
THG FROG
(Third Harmonic) |
Yes |
Yes |
Yes, but not intuitive
|
High |
TG FROG
(Transient Grating) |
High Powered Only |
Yes |
Yes
|
High |
PG FROG
(Polarization Gate) |
No |
Yes |
Yes
|
High |
SD FROG
(Self Diffraction) |
No |
Yes |
Yes
|
Moderate |
Table 1: Various FROG geometries with associated pros and cons.
Each geometry has certain advantages as well as limitations which need to be considered within the context of the laser pulse to be measured. In Application Note 33, we focus on two geometries: self-diffraction FROG (SD FROG) for amplified systems and second harmonic FROG (SHG FROG) for oscillators. The SD FROG trace gives an intuitive picture of the pulse (i.e. the direction of time is preserved), which is a highly desirable feature for real-time laser alignment. Moreover, the geometry is identical to the Long Scan Autocorrelator where the nonlinear medium is a thin piece of glass (<200_m), thus making this geometry cost effective and straightforward to implement. On the downside, SD FROG requires relatively high peak powers not readily available from most ultrafast oscillators, so its use is restricted to amplified systems. For oscillators, the only real option is SHG FROG.i An SHG FROG is simply a spectrally resolved autocorrelator. Although the sign of the spectral phase is lost in the measurement, efficient algorithms exist which can deduce the order and magnitude of the spectral phase where the sign can be determined if need by performing additional measurements.i
Diagram of SHG FROG Setup.
Diagram of SD FROG Setup.
i Rick Trebino, "Frequency-Resolved Optical Gating: The Measurement of Ultrafast Laser Pulses", Norwell, MA, Kluwer Academic Publishers (2000)
* The software included with this product is for reference only. It has been tested in the experimental setup described in the application note. The software will need to be customized by the user for additional experimental requirements. At the present time, Newport provides limited support for this software and any concern should be discussed with Newport before purchasing the product. Please contact our sales representative at toll free number 1-800-222-6440 or email at tech@newport.com to know more about this product.
