In this study, we present simulation studies of an intensity modulation technique to produce an optical pulse train. This technique used two optical phase modulators connected to a Fabry-Perot filter (with a free spectral range of 200 GHz and finesse of 500) connected in series to a fiber. Short RF pulses with a phase difference of 180 degrees induced the optical phase modulators. A digital phase shifter created a 180-degree phase difference between RF signals. In the simulation studies, the technique was investigated in a ring-cavity laser design model at the laser wavelength of 1300 nm. Optical pulses were obtained at a repetition rate of 100 MHz with a pulse width of about 600 ps. It was possible to change the optical pulse parameters by changing the bit sequence configuration and bit-rate of the digital phase shifter. With further improvement in effective control of rapid phase shift between RF signals, the technique can be used in active modelocking.
This study presents a time-stretched wavelength-swept laser source based on stretched-pulse mode-locking. A broadband semiconductor optical amplifier (SOA) technology is used as an optical gain element. The laser comprises a unidirectional ring cavity with matched positive and negative continuously chirped fiber Bragg gratings (FBG’s). One FBG generates a total positive dispersion of 454 ps/nm at 1275 nm and the other chirped FBG generates a total negative dispersion of -454 ps/nm at 1275 nm. A high-extension lithium-niobate intensity modulator (>30dB extinction at 1275 nm, 4.9 dB loss at maximum transmission) is driven with short pulses by a bit pattern generator providing approximately 0.235 ns full-width at half-maximum pulse profiles. These pulses are stretched, amplified, and compressed within the ring cavity, and the modulator pulsing is synchronized to a harmonic of the cavity round trip time. The laser output is provided from the cavity by a 25% coupler. The output light is amplified by another SOA. The laser source provides a sweeping range of approximately 90 nm centered at around 1275 nm at a repetition rate of ~5 MHz. This yields an estimated axial resolution of 8 μm in air.
The vagus nerve originating from the brainstem in the central nervous system is a long cranial nerve that reaches the neck, thorax, abdomen, and colon. It plays a role in autonomic nervous, cardiovascular, gastrointestinal, and immune systems. Electrical stimulation of the vagus nerve has become a standard method for the treatment of neuropathic pain and epileptic conditions over the years. Infrared laser nerve stimulation (ILNS) is an evolving technique that uses infrared laser energy to stimulate cells with electrochemical capacity without the need for external agents or physical contact. This pilot study explores infrared laser stimulation of the rat vagus nerve, in-vivo. An infrared pigtailed singlemode diode laser operating at 1505 nm in continuous-wave (CW) mode was used in this study for noncontact CW-ILNS. Successful CW-ILNS of the rat vagus nerve was observed after the CN reached a threshold temperature of ~44°C with response times as short as 10 s. With more improvement in instrumentation, better optimization of stimulation parameters, and a higher sample size, CW-ILNS may show some potential in vagus nerve stimulation for preclinical.