Filter chirp

The basic unit of the radar waveform is the impulse response h(n) of the transmit filter. It is expedient to view this response in the time domain as an FM chirp signal with envelope a(n) and phase ty(n)... 

We have undertaken an experiment to try to improve the performance of pulse amplifier experiments. The system is shown schematically in figure 2. It consisted of a continuous-wave C102 dye laser amplified in three stages by a frequency tripled Q-switched NdtYAG laser. The output energy was approximately 2.0 mJ in a 150 MHz linewidth and was up-shifted from the continuous-wave laser by 60 MHz caused by the frequency chirp. This light was then spectrally filtered in a confocal interferometer with a finesse of 40 and a free spectral range of 300 MHz. The linewidth of the filtered radiation was approximately 16 MHz. 

The novel aspect of this experiment was that the confocal cavity was locked to continuous-wave radiation which was frequency shifted by an acousto-optic modulator such as to centre the filtering cavity onto the chirped amplified radiation. This reduced the residual amplifier shift to -2(1 MHz. The dominant contribution to this shift resulted from the cw light being injected off-axis into the cavity. Because the filter cavity had a high finesse we used a phase modulation scheme for locking. Indeed, we normally locked the dye laser to the filtering cavity and scanned the spectrum by scanning the filter cavity. 

Figure 6 Block diagram of the two-color optical parametric amplifier (OPA) and IR-Raman apparatus. CPA = Chirped pulse amplification system Fs OSC = femtosecond Ti sapphire oscillator Stretch = pulse stretcher Regen = regenerative pulse amplifier SHGYAG = intracavity frequency-doubled Q-switched Nd YAG laser YAG = diode-pumped, single longitudinal mode, Q-switched Nd YAG laser KTA = potassium titanyl arsenate crystals BBO = /J-barium borate crystal PMT = photomultiplier tube HNF = holographic notch filter IF = narrow-band interference filter CCD = charge-coupled device optical array detector. (From Ref. 96.)...

Figure 5. (A) Scheme of two-photon laser scanning microscope (1) Ti Sa laser, 100 fs, 80 MHz, 750-980 nm, 1.6W 800 nm (TSUNAMI, Spectra Physics), (2) pre-chirp, (3) beam multiplexer, (4) scanning mirrors, (5) microscope (Olympus IX 71, XLUMPLFL20XW, WD = 2 mm, NA = 0.95), (6) fluorescent foci in sample, (7) filter wheel/spectrograph (SpectraPro 2300i, Acton Research Corporation)/spectrometer (home built), (8) back illuminated EMCCD camera (IXON BV887ECS-UVB, Andor Technology), (9) dichroic mirror (2P-Beamsplitter 680 DCSPXR, Chroma). (B) Experimental setup of two-photon laser scanning microscope.

Our experimental setup of the 2D-CARS microscope is shown in Fig. 5.4h [32], The 70-fs output from the Ti sapphire oscillator was split into two beams. One of the beams was introduced into a photonic crystal fiber (Crystal Fibre, Femtowhite 800) to generate a coherent supercontinuum. Then, the continuum was conditioned with an 800 nm long-pass filter. The other beam was spectrally narrowed by the custom-made laser line filter (Optical Coatings Japan, Av = 14 cm FWHM). The obtained two beams were introduced collinearly into the microscope objective lens. The chirping of the broadband pump beam was carefully avoided so that the CARS signals are obtained in a wide spectral region. The broadband CARS emission in back-reflected direction was analyzed by the CCD spectrometer. 

Figure 8.38. Regions of 400 ms NOESY spectra recorded (a) without and (b) with the inclusion of the zero-quantum filter shown in Fig. 8.37. The ZQC suppression employed a 20 ms adiabatic smoothed CHIRP pulse with a 40 kHz frequency sweep.

Blais, S. R. Yao, J. P. (2008). Tunable photonic microwave filter using a superstructured FBG with two reflection bands having complementary chirps. leee Photonics Technology Letters, Vol. 20, pp. 199-201, Issn 1041-1135. 

Capmany, J., Pastor, D. Ortega, B. (1999). New and flexible fiber-optic delay-line filters using chirped Bragg gratings and laser arrays. leee Transactions on Microwave Theory and Techniques, Vol. 47, No. 7, pp. 1321-1326, Issn 0018-9480. 

Hunter, D. B. Nguyen, L. V. T. (2006). Widely tunable RF photonic filter using WDM and a multichannel chirped fiber grating. leee Transactions on Microwave Theory and Techniques, Vol. 54, No. 2, p>p. 900-905, Issn 0018-9480. 

Ning, G., Shum, P. Zhou, J. Q. (2007). Chromatic dispersion effect on microwave photonic filter with a tunable linearly chirped fiber bragg grating. Microwave and Optical Technology Letters, Vol. 49, No. 9, pp. 2131-2133, Issn 0895-2477. 

The appHcations of SAW devices are numerous, their implementation as deflectors, tunable filters, and frequency shifters, as described, are just a few of their possible uses. Other important device applications are time multiplexer, pulse compression of chirped signal, correlators, spectrum analyzer, and isolators. 

In a similar way, geometric decorrelation can also be reduced by bandwidth filtering, at a cost of resolution (Gatelli et al. 1994). This is possible because changing the look angle slightly is approximately equivalent to shifting the chirp frequency. 

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