Terahertz wave modulator based on optically controllable metamaterial

We demonstrated experimentally a terahertz wave modulator based on optically controlled metamaterial. The signal modulation mechanism of the presented terahertz wave modulator was based on the resonance characteristic of metamaterial controlled without or with light excitation. A modulated semiconductor laser with 808 nm wavelength was employed to light the substrate. The interaction between the metamaterial and terahertz wave was strengthened and yielded an appreciable modulation of the terahertz output beam. The modulation speed is 0.1 Kb/s and the modulation depth of the proposed terahertz modulator is about 57% at a frequency of 0.32 THz.     

56 dB Gain EYDFA with improved noise figure with dual-stage partial double pass configuration

An efficient erbium/ytterbium co-doped fiber amplifier (EYDFA) is demonstrated by using a dual-stage partial double pass structure with a band pass filter (BPF). The amplifier achieves the maximum small signal gain of 56 dB and the corresponding noise figure of 4.66 dB at 1536 nm with an input signal power and total pump power of ?50 dBm and 140 mW, respectively. Compared with a conventional single-stage amplifier, the maximum gain enhancement of 16.99 dB is obtained at 1544 nm with the corresponding noise figure is improved by 2 dB. The proposed amplifier structure only uses a single pump source with a partial double pass scheme to provide a high gain and dual-stage structure to provide the low noise figure.     

Dual-stage L-band erbium-doped fiber amplifier with distributed pumping from single pump laser

A dual-stage L-band erbium-doped fiber amplifier with a flat gain bandwidth over 36 nm is demonstrated using pump distribution technique. The pump power was distributed to two stages depending on the splitting ratio and the length of erbium-doped fiber that was used for this configuration. Both parameters are the key components for achieving a substantially flat gain response throughout the L-band region ranging from 1570 nm to 1605 nm. Although the input signal power was varied from ? 30 dBm to 0 dBm, gain of 17 dB with slight variations of less than 1.5 dB and a noise figure of less than 6.7 dB were achieved. All the results obtained show better performances when comparison was made with the conventional single-stage L-band optical amplifier.     

A low transmission loss THz polarization splitter based on dual-core optical fiber

We propose a design of a low loss terahertz polarization splitter based on a dual-core terahertz fiber with crossed dielectric strips in the fiber cross section. Low transmission loss is realized by extending the mode field to the air holes adjacent to the solid material. An 11.4-cm-long terahertz polarization splitter is obtained with the extinction ratio better than −15 dB and a bandwidth of 16 μm.     

Cascaded continuous-wave singly resonant optical parametric oscillator pumped by a single-frequency fiber laser

We present a cascaded continuous-wave singly resonant optical parametric oscillator (SRO) delivering idler output in mid-IR and terahertz frequency range. The SRO was pumped by an ytterbium-doped fiber laser with 27 W linear polarization pump powers, and based on periodically poled MgO:LiNbO₃ crystal (PPMgLN) in two-mirror linear cavity. The PPMgLN is 50 mm long with 29.5 μm period. The idler power output at 3811 nm was obtained 2.6 W. The additional spectral components that have been attributed to cascaded optical parametric processes are described at increasing pump levels. Besides the initial signal component at about 1476.8 nm, further generated wavelengths with frequency shifts about 47 cm^?1, 94 cm^?1 and 104 cm^?1 were observed. It was speculated that the idler waves lie in the terahertz (THz) domain from the observed results.     

Wide-field imaging using a tunable terahertz free electron laser and a thermal image plate

The appearance of intense terahertz sources such as quantum cascade laser and free electron laser opens up new opportunities for 2D imaging. Though microbolometer and pyroelectric arrays are promising recorders, they are of small size and cannot be used when wide-field imaging in the longwave region is required. We applied for terahertz imaging 3″ × 3″ and 6″ × 6″ Macken Instruments Inc. “thermal image plates”, a set of thermal sensitive phosphor screens operating in a room temperature environment. The Novosibirsk free electron laser was used as a source of radiation. We have found that the response of thermal image plate is linear until the relative quenching is less than 60% of the initial luminescence intensity. The response curve follows the Seitz–Mott law. The threshold sensitivity was found to be 100 mW/cm² at 1.5 THz and 40 mW/cm² at 2.3 THz. Interferograms, holograms, and terahertz beam spatial distributions recorded in the spectral range of 1.2–2.5 THz are given as examples.     

Optical single sideband-frequency generation with carrier totally suppressed for Brillouin distributed fiber sensing

The power fluctuation and multi-sidebands of the RF modulated electro-optic intensity modulator (EOIM) can lead to a degraded resolution of the Brillouin distributed sensors. A standard commercial fiber-pigtailed DFB-LD is locked to one of the sidebands of the EOIM. Optical single sideband-frequency generation is realized with the carrier and the other sidebands totally suppressed. The locked LD greatly improves the power stability of the injected sideband. No dc bias controlling to the EOIM is needed in the locking process. This simple method provides large tuning range (～1 GHz), fast response (～100 μs), and high side-frequency suppression ratio (～60 dB). Thus, it is practical for high-resolution Brillouin sensing systems as well as microwave photonics.     

Terahertz emission dependence on the intensity ratio of 400–800 nm in generating terahertz waves from two-color laser-induced gas plasma

A transient photocurrent model is used to explain terahertz emission from gas plasma irradiated by a laser pulse and the second harmonic. By introducing the second harmonic, 400 nm, the corresponding terahertz emission is greatly enhanced. The exact dependence of terahertz emission on the intensity ratio of 400–800 nm is studied for the case with total intensity of 5.00 × 10¹⁴ W/cm². Results show the emission reaches the maximum at about the case for energy distribution of I_ω = 4.00 × 10¹⁴ W/cm², I_2ω = 1.00 × 10¹⁴ W/cm².     

A novel triple pump 1050 nm, 1400 nm, 800 nm pumping scheme for thulium doped fiber amplifier

Thulium doped fiber amplifier is a good candidate for S, and S+ band. This paper demonstrated a three pump pumping scheme for thulium doped fiber amplifier with 1050 nm co propagating pump and 1400 nm and 800 nm counter propagating pumps with a total pumping power 600 mW. This configuration yields up to 33 dB gain in 20 nm region from 1460 nm to 1480 nm, with noise figure <4 dB. To the knowledge of authors it is the highest gain achieved by thulium-doped amplifier in a single pass configuration with good power conversion efficiency.     

A phase stable short pulse generator using a DPMZM and phase modulators for application in 160 GBaud DQPSK systems

A method of 40 GHz phase stable short pulses generation is experimentally demonstrated. It is based on a dual parallel Mach–Zehnder modulator (DPMZM) driven by only one electrical sinusoidal clock and two cascaded phase modulators. The generated pulses are characterized with full-width-at-half-maximum pulse width of 1.9 ps, extinction ratio of 27 dB, timing jitter of 36 fs and signal to noise ratio over 30 dB. The high quality and phase stability of the pulses are further experimentally verified in a 4 × 40 GBaud differential quadrature phase shift keying (DQPSK) optical-time-division-multiplexing (OTDM) system.     

Design of a super wide-band EM wave absorber for a general purpose anechoic chamber

In order to construct an anechoic chamber satisfying international standards for EMI testing, it has been recognized that the absorption characteristics of the EM wave absorber must be higher than 20 dB over the frequency band from 30 MHz to 18 GHz. In this paper, an EM wave absorber with super wide-band frequency characteristics is proposed and designed in order to satisfy the above requirements by using the EMCM and FDTD. As a result, the proposed absorber has absorption characteristics higher than 20 dB over the frequency band from 30 MHz to more than 20 GHz.     

Femtosecond single-mode diode-pumped Cr:LiSAF laser mode-locked with single-walled carbon nanotubes

A low threshold Cr:LiSAF laser pumped with an inexpensive single-mode laser diode emitting 120 mW was passively mode-locked with a novel ultrafast saturable absorber mirror based on single-walled carbon nanotubes (SWCNT-SAM). Pulses as short as 122 fs were achieved, tunable across 14 nm. A second pump diode coupled in polarization allowed to shorten the pulse duration to 106 fs, with up to 24-mW output power.     

A new design of low-loss and ultra-flat zero dispersion photonic crystal fiber using hollow ring defect

New hollow ring defect structure is introduced in photonic crystal fiber design for ultra- flat zero dispersion with very low waveguide losses. The hollow ring defect consisted of a central hole surrounded by a doped silica ring provides highly flexible defect engineering capabilities in photonic crystal fibers to achieve precise control of dispersion value and dispersion slope while independently maintaining low waveguide losses, which was not attainable in previous designs. A nearly flat zero dispersion of D=0±0.51 ps/nm km was obtained in the wavelength range of 1.44–1.61 μm with the maximum slope of ?2.7×10^?2 ps/nm² km. The confinement loss was less than 5.75×10^?8 dB/m along with the bending loss of 2.8×10^?6 dB/m for the radius of 10 mm, and splice loss of less than 1.57 dB to conventional single mode fiber at 1.55 μm.     

Semiconductor type single wall carbon nanotube absorber for passive mode-locked Nd:YVO4 laser

The pure semiconductor type single wall carbon nanotubes (SWCNT) was transferred on hydrophilic glass substrate to fabricate saturable absorbers by vertical evaporation technique. The recovery time of the absorber is 350 fs. The saturation intensity of the absorber was found to be 115 μJ/cm² at 1060 nm. The modulation depth of the absorber could be about 7%. Passive mode-locked Nd:YVO₄ laser using this kind of absorber was demonstrated. The largest average output power of the mode-locked laser is 1.4 W at the pump power of 7.8 W. The continuous wave mode-locked pulses with the repetition of 80 MHz were achieved.     

Adjustable EDFA gain equalization filter for DWDM channels based on a single LPG excited by flexural acoustic waves

A fast adjustable gain equalization filter for dense wavelength division multiplexing (DWDM) system is reported. The method is based on a single long period fiber grating (LPG) which is excited by means of flexural acoustic waves. The equalization of a typical erbium doped fiber amplifier (EDFA) gain spectrum with a gain flatness of <0.3 dB over a 32 nm bandwidth is demonstrated. The filter adjustment is obtained by choosing different acoustic loads applied to the acousto-optic modulator, which presents a switching time of ～17 μs. A maximum power penalty of 0.84 dB, relatively to the back-to-back signal, was achieved.     

Magnetically tunable silicon-ferrite photonic crystals for terahertz circulator

The gyromagnetic properties of ferrite materials and the nonreciprocal property of a silicon-ferrite photonic crystal cavity are investigated in the terahertz region. Through the structure optimization and analysis of defect mode coupling, we design a magnetically tunable circulator, of which central operating frequency can be tuned from 180 to 205 GHz and the maximum isolation is 65.2 dB. Moreover, the further study shows that the gyrotropy, dispersion, and ferromagnetic loss of ferrite materials under the different external magnetic fields greatly affect the transmission and isolation property of this device. This circulator is flexible to realize functions of controllable splitting, routing, filtering and isolation by changing the external magnetic field for the THz applications.     

High index contrast Er:Ta2O5 waveguide amplifier on oxidised silicon

We report a high index contrast erbium doped tantalum pentoxide waveguide amplifier. 2.3 cm long waveguides with erbium concentration of 2.7 × 10²⁰ cm^? 3 were fabricated by magnetron sputtering of Er-doped tantalum pentoxide on oxidised silicon substrates and Ar-ion milling with photolithographically defined mask. A net on-chip optical gain of ~ 2.25 dB/cm at 1531.5 nm was achieved with 20 mW of pump power at 977 nm launched into the waveguide. The pump threshold for transparency was 4.5 mW.     

Modeling of laser-diode side-pumped continuous wave Nd:YAG/KTP green laser

By reasonably assuming the distribution of the pump light in laser medium, the equations about the absorption and the gain for the end-pumped lasers are applied to the side-pumped ones, and a theoretical model for laser-diode side-pumped continuous wave intracavity-frequency-doubling lasers is given, in which the thermally induced diffraction loss and the variation of the fundamental wave radii with the pump power are considered. By using a Z-type cavity, a laser-diode side-pumped continuous wave Nd:YAG/KTP green laser is realized. The threshold pump power is 15 W, and the highest output power of the green laser is 3.75 W at the pump power of 160 W, corresponding to an optical–optical slope efficiency 2.6%. The experimental results are consistent with the numerical solutions.     

Inherently gain flattened S-band erbium doped fiber amplifier based on dual core resonant leaky fiber

A co-axial dual core resonant leaky optical fiber (DCRLF) is designed for inherent gain equalization of S-band erbium doped fiber amplifier (EDFA). Resonance tail of leakage loss of the fiber into the S-band region is utilized to flatten the gain. We have numerically studied the effect of various design parameters and their fabrication tolerances on gain flattening. We show 23.5 dB flat gain with ± 0.9 dB ripple over 30 nm bandwidth (1490–1520 nm) using 120 mW pump. The study should be useful in designing optical fiber amplifiers for optical communication system employing wavelength division multiplexing.     

A novel OCS millimeter-wave generation scheme with data carried only by one sideband and wavelength reuse for uplink connection

We propose a novel optical carrier suppression (OCS) millimeter-wave generation scheme with data carried only by one sideband using a dual-drive Mach–Zehnder modulator (MZM) in radio-over-fiber system, and the transmission performance is also investigated. As the signal is transmitted along the fiber, there is no time shifting of the codes caused by chromatic dispersion. Simulation results show that the eye diagram keeps open and clear even when the optical millimeter-waves are transmitted over 110 km and the power penalty is about 1.9 dB after fiber transmission distance of 60 km. Furthermore, due to the +1 order sideband carrying no data, a full duplex radio-over-fiber link based on wavelength reuse is also built to simplify the base station. The bidirectional 2.5 Gbit/s data is successfully transmitted over a 40 km standard single mode fiber with less than 0.8 dB power penalty in the simulation. Both theoretical analysis and simulation results show that our scheme is feasible and we can obtain a simple cost-efficient configuration and good performance over long-distance transmission.