## 9 Challenge: Double Rainbow!--This Is a True Story
They were equally spaced in frequency, and were extended from the far-UV to the NIR! I was wondering whether I should call you immediately to look for yourself or not.\"  
\"When did you find them?\"  
\"Two o'clock\", he said. \"AM or PM?\"  
\"AM\", he answered. (I thought I was happy not to have received his phone call at such an early time.) \"How were you able to obtain this result?\" I asked to him. \"Today, I chirped the laser pulse for stretching.\"  
Later, we noticed that the laser was operating under improper conditions: the laser had a slightly narrow spectral bandwidth (longer pulse width), although the engineer from the manufacturer had suggested that we should not operate the system under such unfavorable conditions because this might cause serious damage to the laser. In fact, we frequently destroyed expensive Ti:sapphire rods in the experiment. However, such a condition was favorable for suppressing undesirable effects such as self-phase modulation and self-focusing. We also noticed that the NIR laser was useful for the exclusive generation of rotational Raman emissions. **11. Dream: Molecular-Optic Modulator**  
One day, I was thinking about a laser cavity. I noticed that a laser consisting of equally-spaced frequencies can transmit through the cavity, and that the intensity of the light can be strongly enhanced. If this is correct, all rotational Raman emissions can be resonated in the cavity under certain conditions. This consideration suggests that Rainbow Stars--a type of nonlinear optical effect--can be generated even using a continuous-wave (CW) low-intensity laser. This idea was published in a scientific journal and was also patented. In order to increase the intensity of the light in the cavity, it is necessary to increase the finesse of the cavity using a pair of mirrors with a high reflectivity (e.g., 99.98 %), and to use a monochromatic laser beam with a narrow linewidth (e.g., 100 kHz). We solved numerous problems, and were able to match the laser beam and the cavity after a 5-year effort! Eventually,we were able to generate a Stokes beam using a CW Ti:sapphire laser. An optical beat formed by the fundamental beam, and the Raman beam was measured using a laboratory-made autocorrelator. Based on current technology, it is possible to modulate a laser beam in the MHz region using an acousto-optic modulator (AOM) and in the GHz region using an electro-optic modulator (EOM). We were able to modulate the laser beam in the THz region based on molecular motion. We referred to this device as a \"molecular-optic modulator (MOM)\". In order to generate a train of pulses, it is necessary to generate more than three emission lines. By carefully adjusting the laser wavelength and the cavity length, it was possible to generate the second Stokes beam, in addition to the fundamental and the first Stokes beam. A train of optical pulses, generated at 17.6 THz, was measured using the autocorrelator. **12. Obsession: Dispersion-Compensation in the Cavity**  
The spacing among the three emission lines which consist of the fundamental and two Stokes beams cannot be exactly equal, because the refractive index changes with changing wavelengths. For this reason, the waveform of the pulse changes at different times. In order to solve this problem, it is necessary to use a coherent process such as four-wave Raman mixing (FWRM). However, generating an anti-Stokes beam by FWRM is a difficult task due to unavoidable positive dispersion of hydrogen in the cavity. To address this issue, we developed a dispersion-compensated optical cavity for anti-Stokes generation. A pair of mirrors with negative dispersions were used, and the total dispersion in the cavity was adjusted to zero by optimizing the hydrogen pressure and by adding a noble gas with different optical properties. This was not an easy task, because the cavity length, the laser wavelength, and the pressures of the hydrogen and xenon gases must be very carefully changed in a stepwise manner and be precisely optimized.