Virtual Journal of Laser

Optical beam steering can find applications in several domains such as laser scanning, LiDAR (Light Detection And Ranging), wireless data transfer and optical switches and interconnects. As present beam steering approaches use mechanical motion such as moving mirrors or MEMS (MicroElectroMechanical Systems) or molecular movement using liquid crystals, they are usually limited in speed and/or performance. Therefore we have studied the possibilities of the integrated silicon photonics platform in beam steering applications. In this paper, we have investigated a 16 element one-dimensional optical phased array on silicon-on-insulator with a field-of-view of 23
$^{circ}$. Using thermo-optic phase tuners, we have shown beam steering over the complete field-of-view. By programming the phase tuners as a lens, we have also shown the focusing capabilities of this one-dimensional optical phased array. The field-of-view can easily be increased by decreasing the width of the waveguides. This clearly shows the potential of silicon photonics in beam steering and scanning applications.

We report an experimental investigation on remote transfer of a femtosecond-laser frequency comb through an open atmospheric link. Optical multiheterodyne is used to measure the excess phase noise and the frequency stability of the transferred comb. The dispersion of air is found to have a minimal impact on the multiheterodyne signal, and the effectiveness of the technique to characterize the behaviors of comb lines under the influence of turbulence is theoretically analyzed. Large phase modulation due to the index fluctuation of the air over a 60-m transmission link is found to cause a significant linewidth broadening. Under low-wind conditions, a fractional frequency stability in the order of
$10^{-14}$ has been achieved over several minutes with a 1-s averaging time. A comparison of this work with previous tests based on continuous wave (CW) lasers indicates that pulsed lasers can work as well as CW lasers for remote transfer of optical frequency references through the atmosphere.

The phase response of an injection locked semiconductor laser that is used as the phase modulator in a resonant cavity linear interferometric intensity modulator is studied in detail. It is shown that, signal-to-intermodulation ratio of such a modulator is affected by the injection ratio, linewidth enhancement factor of the semiconductor laser, residual amplitude modulation, depth of phase modulation, and linearity of the resonant cavity response. Experimental measurements of the signal-to-intermodulation ratio of this modulator using a semiconductor Fabry-Pérot laser as the resonant cavity are in good agreement with the theoretically predicted values.

A center frequency-tunable multi-tap bandpass microwave photonic filter (MPF) is proposed and experimentally demonstrated. A specially designed multi-wavelength fiber ring laser, in which a windowed Fabry-Pérot (FP) filter is used as the wavelength selection and power control component, has been developed to serve as the optical source for the MPF. By adjusting the windowed FP filter, both the wavelength spacing and power profile of the multi-wavelength laser can be changed. The output of the optical source is phase modulated by a microwave signal. 25 km of single-mode fiber (SMF) is then used to act as a dispersive medium to introduce time delays between taps. Thus, a tunable bandpass response is obtained at the output of a high-speed photodetector (PD). In addition, the passband centered at DC is removed due to the use of phase modulation. The experimental results show that more than 45 wavelengths can be generated in the multi-wavelength ring laser. With the electronic tuning of the wavelength spacing, tuning of the passband center frequency of the MPF by 3 GHz is achieved.

A hybrid mode-locked Erbium-doped fiber laser that provides very short pulse-widths while achieving high repetition rates is proposed and experimentally demonstrated. This hybrid configuration is realized by using a thin film of polydimethylsiloxane (PDMS) doped with single wall carbon nanotubes (SWCNT). This PDMS/SWCNT composite acts as a saturable absorber and is inserted within an active mode-locked laser system using angled connectors. Therefore, the effect of the PDMS/SWCNT composite is to effectively narrow the width of the pulses generated by the active system without modifying its repetition rate. A pulse-width of 730 fs was generated at a repetition rate of 4 GHz, while achieving an average output power of 4 mW. A reduction in the noise of the photodetected RF spectrum was also observed in the hybrid system.

Optical frequency combs are used as local oscillators for the measurement and analysis of unknown optical waveforms with periodic time domain structures. Experimental results obtained by heterodyning pulsed and phase-modulated laser sources are presented. The analysis is then extended to the heterodyning and sampling of bandlimited incoherent light sources. It is shown theoretically and experimentlly that the correlations between photodetected white light at different times can generate RF interference that is sensitive to the optical phase.

A two-bit all-optical digital comparator using single mode Fabry–Pérot laser diodes (SMFP-LDs) at an input data rate of 10 Gbps is proposed and demonstrated. All-optical comparator is demonstrated using cascaded logic units which are based on injection locking, multi-input injection locking and supporting beam principles for suppressing the dominant mode of SMFP-LDs. Digital comparators are the key components for the decision making circuits, the integral part of the arithmetic and logical units of optical data processors. The output performance of the proposed all-optical comparator is verified with output waveform, rising-falling time, output eye diagram, and bit error rate (BER) at 10 Gbps input Non Return to Zero (NRZ) PRBS of
$2^{31}-1$ signal. The rising/falling time of about 47 ps, clear output waveforms, and clear output eye diagram with an extinction ratio of about 12 dB are obtained. A maximum power penalty of 1.3 dB is measured at a BER of
$10^{-9}$.

The application of spectroscopic ellipsometry for the characterization of UV-patterned channel waveguides to obtain the refractive index contrast and surface deformation profile is presented. Thin films were prepared with organic–inorganic di-ureasils hybrids modified with zirconium tetra-propoxide deposited in silica on silicon substrates. The channel waveguides were produced by direct writing using UV laser radiation. The refractive index contrast and the surface ablation induced by the UV optical signal were estimated by ellipsometry being
$4.5 times 10^{-3}$ and 30.5 nm, respectively. The deepness of the surface ablation due to the UV exposition was also estimated by atomic force microscopy measurements that pointed out a value of
$31.0 pm 1.0$ nm, concordant with the ellipsometric calculations. The near-field intensity technique was used as a support for contextualizing the proposed ellipsometry method for the characterization of refractive index profiles.

We present an equivalent circuit cut-off frequency analysis of a Coupled-Cavity Vertical-Cavity Surface-Emitting Laser with one cavity used as a fast electro-optic modulator with lumped electrodes. We find that the mesa capacitance and the polyimide capacitance and series resistance are the most influencing parameters. In order to enhance the
${-}3$ dB bandwidth the design steps should be: reduction of the contact pad area and modulator mesa radius together with increasing the modulator cavity length, as well as implementing a pnp structure. Based on the model, we suggest a realistic structure that is theoretically able to achieve about 100 GHz modulation speed.

A novel technique to achieve ultrafast and ultrahigh-resolution interrogation of a fiber Bragg grating (FBG) sensor based on interferometric temporal spectroscopy is proposed and experimentally demonstrated. In the proposed system, two FBGs with one serving as the sensor grating and the other serving as the reference grating are connected at two arms of an interferometer. An ultrashort optical pulse from a pulsed laser is sent to the interferometer. Two pulses will be obtained due to the reflection of the two FBGs and then both are sent to a dispersive element to map the sensor grating wavelength shift to a temporal spacing change between the two dispersed pulses due to the dispersion-induced wavelength-to-time mapping. A temporal interference pattern is generated between the temporal pulses. The temporal spacing change is further mapped to the interference pattern frequency change, leading to a greatly improved interrogation resolution due to the inherently high sensitivity of a temporal interferometer.