We demonstrated single-shot measurements of spectral interference between a white-light continuum generated in a hollow-fiber and its second harmonic. The interference has information on the carrier-envelope phase of an input pulse to the fiber and the time delay of the blue wing of the continuum. By analyzing the observed spectral interference, we estimated shot-by-shot changes of the carrier-envelope phase. This method is useful for determining the carrier-envelope phase changes of a low-repetition-rate, high-intensity laser.

We present systematic experimental studies of high-order harmonic generation in He and Ne that have been performed with an intense, femtosecond Ti:sapphire laser at intensities in the tunneling regime, where special attention is concentrated on the strong-field effect. A broad plateau is formed in the harmonic distribution that includes harmonic peaks at low and high orders and the observed highest orders (shortest wavelengths) are the 103rd (7.6 nm) for He and 95th (8.2 nm) for Ne. The laser-ellipticity-dependent harmonics observed suggest different generation processes for the low orders below and around the atomic ionization limit and for the high orders in and above the plateau region. The medium ionization is found to induce strong self-defocusing of the incident laser beam, and its effects on the harmonic generation are studied. The results for the defocusing and the blueshifts of the harmonic spectra observed show that the generation of a higher-order harmonic requires a higher intensity in time and space. The theoretically predicted harmonic cutoff is demonstrated to agree well with experiment when we take into account the interaction intensity limited in the ionizing medium. The characteristic harmonic behaviors observed are discussed based on the interaction intensity and the propagation effect in the medium. PACS number(s): 32.80.Rm; 42.65.Ky

A frequency comb spanning more than one octave has been achieved by injecting the second-harmonic generation (780 nm) of a mode-locked fiber laser (1.56 microm) into a photonic crystal fiber. We propose and realize a novel interferometric scheme for observing the carrier-envelope offset frequency of the frequency comb. Frequency noise has been observed on the measured carrier-envelope offset frequency, which has been confirmed to be generated in the photonic crystal fiber by comparing the measured beat frequencies between cw lasers and frequency combs before and after the photonic crystal fiber. The mode-locked fiber laser is considered to be an important candidate for the light source used in realizing a compact optical frequency measurement system including applications in the telecommunication bands.

No abstract

No abstract