Virtual Journal of Laser

A polarization-independent, multifunctional all-fiber comb filter by using variable ratio coupler (VRC)-based dual-pass Mach–Zehnder interferometer is newly proposed and demonstrated. Depending on the dynamic settings of coupling ratios of the fiber couplers, the comb filter provides the channel spacing tunable, multiwavelength switchable, and flat-top spectral responses. Theoretical calculations are verified by the experimental results. Experimentally, a comb filter with the channel spacing tunable from 0.4 to 0.8 nm was obtained. Meanwhile, when the comb spacing was tuned to 0.8 nm, the multiwavelength switchable operation with both the sinusoid and flat-top spectral responses was achieved. The proposed comb filter may find applications in optical communication systems and tunable multiwavelength fiber lasers.

We demonstrate a cost-effective, 10-Gb/s full-duplex wavelength-division-multiplexed passive optical network over 20 km single-feeder fiber using a single light source per optical network unit (ONU). We exploit a directly modulated laser (DML) and a reflective semiconductor optical amplifier (RSOA) for downlink and uplink, respectively. In this system, a nonreturn-to-zero (NRZ) downstream signal is first frequency up-converted to around 20 GHz before being fed to a DML for downstream transmission. After transmission over the feeder fiber, a portion of the downstream signal is detected at the ONU and the other portion is fed to the RSOA as seed light. The RSOA is directly modulated by a 10-Gb/s NRZ signal for upstream transmission. To overcome the bandwidth limitation of the directly modulated RSOA, we employ an optical delay interferometer at the upstream receiver. The passive optical device acts as an optical equalizer and enables the 10-Gb/s upstream signal to be accommodated with a 1.3-GHz-bandwith RSOA. We investigate through experiment the crosstalk between the downstream and upstream signals as a function of the downstream subcarrier frequency.

Yb-doped double-cladding photonic crystal fibers have become key components for power scaling in fiber laser systems, by providing many advantages, especially an ultra large effective area. The single-mode regime, which is a mandatory requirement for high quality laser beams, can be obtained in such large core active fibers only through a careful design. In this paper the cut-off properties of 19-cell photonic crystal fibers have been thoroughly investigated with the avoided-crossing approach, in order to find guidelines for the design of single-mode fibers. The air-hole diameter and the core refractive index have been changed, as well as the number of air-hole rings in the fiber inner cladding. Simulation results have shown that, regardless of the air-hole ring number, the guided-mode cut-off properties are strongly influenced by the main design parameters, especially by the core refractive index. In particular, a wider single-mode wavelength range can be obtained in 19-cell fibers with small air-holes and low core refractive index.

As the demand for bandwidth increases, optical interconnects are coming closer and closer to the chip. Optical interconnects on silicon-on-insulator (SOI) are desirable as this allows for integration with CMOS and the mature processing can be used for photonic integrated circuits. A heterogeneous integration process can be used to include III-V active optical components on SOI. For dense integration compact sources and detectors are required, but they typically need different epitaxial structures to be efficient which limits the integration density. We propose to use an epitaxial structure, which contains both the layers for a laser and for a detector, hereby enabling very compact integration of sources and detectors. Microdisk lasers and waveguide detectors using this epi were completely fabricated in a 200 mm CMOS pilot line and the results are discussed here.

This work presents an approach to multiple access for free-space laser communication (lasercom) called space-time division multiple access, which aggregates traffic from multiple users at the network edge. The objective is to share resources to lower the cost, size, weight, and power consumption per user, thereby making lasercom feasible for users that require only moderate average information rates. This concept relies on fast, agile electronic beam steering, which was implemented in this investigation using liquid crystal optical phased arrays. We designed and built an experimental terminal incorporating a bidirectional communication aperture that was shared among the users, and two independently operated acquisition and tracking apertures. Using two remote user terminals, experiments were conducted to measure access node performance for a variety of operating conditions traceable to anticipated applications. The transmit and receive directions of the downlink and uplink communications channels were rapidly hopped between the two users, and data were exchanged between the access node and a user while the optical channel dwelled on the latter.

A novel and flexible photonics-based scheme is proposed for generating phase-coded RF pulses suitable for coherent radar systems with pulse compression techniques. After selecting two modes from a mode-locked laser (MLL), the technique exploits an optical in-phase/quadrature modulator driven by a low-sample rate and low-noise direct digital synthesizer to modulate the phase of only one mode. The two laser modes are then heterodyned in a photodiode, and the RF pulse is properly filtered out. The scheme is experimentally validated implementing a 4-bit Barker code and a linear chirp on radar pulses with a carrier frequency of about 25 GHz, starting from an MLL at about 10 GHz. The measures of phase noise, amplitude- and phase-transients, and autocorrelation functions confirm the effectiveness of the scheme in producing compressed radar pulses without affecting the phase stability of the optically generated high-frequency carriers. An increase in the radar resolution from 150 to 37.5 m is calculated.

Grating couplers for side coupling of the light from a high-power laser diode array (LDA) into the inner cladding of a double-clad fiber were studied theoretically and implemented experimentally. In the experiments, two types of grating couplers were designed and fabricated: a gold-coated surface relief grating coupler and a gold-embedded silica grating coupler. A suitable design for heat dissipation for each grating coupler was employed to minimize thermal expansion due to the heat accumulated from the light absorption by the metal part of the grating coupler. The experimental results show that the gold-embedded silica grating coupler is superior to the surface relief gold grating coupler, because of its higher resistance to thermal expansion and better heat removal capability. In addition, the grating pitch and groove width were optimized for the highest overall coupling efficiency by taking account the backward diffraction loss and the groove wall nonverticality due to fabrication distortion.

We propose and analyze a photonic-assisted periodic triangular-shaped pulses generator, in which a continuous wave (CW) laser is connected to a dual electrode Mach-Zehnder modulator (De-MZM), and then the De-MZM is connected with a normal dispersion fiber or element. A radio frequency (RF) sinusoid directly drives the De-MZM to obtain optical subcarrier modulation with optical carrier suppression. By carefully setting the physical parameters, the generated harmonics of optical intensity can be corresponding to the Fourier components of typical periodic triangular pulses. Numerical simulations reveal that modulation index must be set to 2.305 and bias voltage drift must be within a range from
${-}2.5hbox{%}-1.5hbox{%}$ from the minimum transmission of the De-MZM. In addition, the dispersion must be set correctly to suppress the primary distortion of 4th order harmonics.

This paper presents a three axis slim fiber laser vector hydrophone. Using a V-shaped flexed beam as a mass-spring element, the acceleration in the radius direction can induce axial strain in the fiber laser. Theoretical analysis and finite element method are both used to evaluate the sensitivity and frequent response of the hydrophone. The experimental results show an acceleration sensitivity of 53.2 pm/g, 39.2 pm/g and 38.1 pm/g at the x, y, z axis respectively, a resonant frequency of about 310 Hz, and a directivity larger than 30 dB.

In this work, a novel single-longitudinal-mode (SLM) four-wavelength laser configuration for sensing applications in the L-band is proposed and experimentally demonstrated. The sensor system presented here is based on ring resonators, and employs fiber Bragg gratings to select the operation wavelengths. The stable SLM operation is guaranteed when all the lasing channels present similar output powers. It is also experimentally demonstrated that when a SLM behavior is achieved, lower output power fluctuations are obtained. Characterization of the lasing structure for temperature sensing is also shown.