Spectral-Amplitude-Coded OCDMA Optimized for a Realistic FBG Frequency Response

We develop a methodology for numerical optimization of fiber Bragg grating frequency response to maximize the achievable capacity of a spectral-amplitude-coded optical code-division multiple-access (SAC-OCDMA) system. The optimal encoders are realized, and we experimentally demonstrate an incoherent SAC-OCDMA system with seven simultaneous users. We report a bit error rate (BER) of 2.7times10^-8 at 622 Mb/s for a fully loaded network (seven users) using a 9.6-nm optical band. We achieve error-free transmission (BER<1times10^-9) for up to five simultaneous users     

Generation of frequency-tunable light and frequency reference gridsusing diode lasers for one-petahertz optical frequency sweep generator

Generation of frequency-tunable light and frequency reference grids in a wide frequency span for a diode laser based optical frequency sweep generator has been performed. Frequency tuning and noise characteristics in nonlinear frequency conversions have been discussed. By using AlGaAs, InGaAsP lasers and their frequency conversions in the type II angle phase-matching KTP crystal, highly coherent frequency-tunable outputs have been obtained from 600 THz (0.5 μm) to 170 THz (1.7 μm). Use of the DFB lasers ensures the continuous tuning with a frequency range as wide as 1 THz. Atomic potassium and molecular iodine absorption resonances have been employed as frequency references for stabilizing the frequencies of lasers and the generated light with the frequency stability of 10^-9-10^-10. Optical frequency comb generation has been realized at the 0.8 μm wavelength with a two-sided sidebands span of 4 THz. We have also proposed and demonstrated specific frequency-tunable systems based on sum and difference-generations of diode lasers     

Frequency stabilized ammonia laser system for probing the 24 THzclock transition of a trapped barium ion

The design, performance and frequency stability of an optically pumped ammonia laser operating on the sP(8,6) transition of ¹⁵NH₃ at 12.48 μm wavelength are discussed. The short term frequency stability was tested by heterodyning two similar lasers. An Allan deviation of 2·10^-11 for averaging times from 10^-4 s to 1 s was achieved for the free running laser. Absolute values of the laser frequency were obtained by measuring against the NRC H4 hydrogen maser standard with the help of the NRC frequency chain. A scan across a narrow fine-structure transition in a single trapped barium ion provided a final test for the overall stability     

Frequency stabilization of a novel 1.5-μm Er-Yb bulk laser to a39K sub-Doppler line at 770.1 nm

The second harmonic output at 770.1 nm of a novel and compact Er-Yb:glass laser was frequency stabilized against the sub-Doppler linewidth of a crossover line in the ³⁹K 4S_1/2-4P _1/2 transition as obtained by saturation spectroscopy. Efficient frequency doubling, with a conversion efficiency of ~220% W ^-1, and with second harmonic power in excess of 15 μW, was achieved in a waveguide made in a periodically poled lithium niobate crystal. As measured through the analysis of the closed-loop error signal, a laser frequency instability of ~200 Hz was obtained; the Allan standard deviation of the frequency samples was below 4×10^-12 for integration times τ between 100 ms and 100 s, and reached a lowest floor level of 8×10^-13 for 20 s⩽τ⩽100 s. The measured frequency noise spectral density was in good agreement with the analysis performed in the time domain. Compared to previously published data for stabilized solid-state laser sources in this wavelength region, these results represent a significant improvement in the frequency stability     

Laser-pumped rubidium frequency standards: new analysis andprogress

We have achieved a stability of 3·10^-13 τ ^-1/2 for 3<τ<30 s with a laser-pumped rubidium gas-cell frequency standard by reducing the effects due to noise in the microwave and laser sources. This result is one order of magnitude better than the best present performance of lamp-pumped devices     

Frequency measurements of optically pumped far-infrared laser lines

In this work, we report the frequency measurements of optically pumped far-infrared (FIR) laser lines. We use the heterodyne technique of mixing FIR laser radiation and microwave radiation on a metal-insulator-metal (MIM) point contact tunnel diode, to determine the FIR laser frequencies. The two FIR laser systems, consisting of CO₂ waveguide pump lasers and Fabry-Perot FIR laser cavities, and MIM diode were developed by us. To check the system, we have measured some FIR laser line frequencies previously reported in the literature. An average fractional frequency reproducibility of ±7×10^-7, between our measurements and the previous ones, permit us to use our system to measure five new FIR laser frequencies     

Millimeter wave narrowband optical fiber links using externalcavity semiconductor lasers

We present the theoretical analysis and the experimental implementation of a narrowband millimeter wave optical fiber communication system using an external cavity semiconductor laser. We derive analytic expressions and present experimental data for the modulation response, relative intensity noise, carrier-to-noise ratio, and harmonic distortion for a semiconductor laser in an external cavity operating as a transmitter in the millimeter wave frequency range. We present a system implementation of this capable of transmitting 40-Mbt/s digital data at a 35-GHz subcarrier frequency with bit-error rates below 10^-9 over a 6.3-km-long optical fiber link     

Short-term stability of laser-pumped rubidium gas cell frequencystandard

The authors examine the influence of the light-shift effect on the frequency stability of a laser-pumped Rb gas cell standard for averaging time up to 10⁴ s. The measured stability at the flicker floor is 7×10^-14 for averaging times from 200 to 5000 s, providing that the laser frequency is properly set at the optical resonance frequency, and correcting for the variation of atmospheric pressure     

飞秒光学频率梳在精密测量中的应用

飞秒光学频率梳通过锁定飞秒锁模激光的重复频率和偏置频率至微波频率基准,在时域上得到重复频率稳定的飞秒脉冲激光,在频域上得到频率间隔稳定的激光频率梳。飞秒光学频率梳作为微波频率与光学频率的桥梁,可以实现对激光频率的直接精密计量,同时作为一种有别于传统连续波稳频激光的特殊激光光源,在激光频率标尺、绝对距离测量和精密光谱测量等光学精密测量领域都有着重要应用。综述了飞秒光学频率梳在若干光学精密测量应用中的研究进展、关键技术和研究动向,分析了其在未来光学测量中的重要作用。     

Laser diode optical frequency stabilisation technique on ITU-T frequency grid employing modulated sideband light

A novel optical frequency stabilisation technique on the ITU-T frequency grid employing modulated sideband light is proposed. The L-band laser diode second sideband light generated by phase-modulation is locked to a carbon monoxide ¹²C¹⁶O gas absorption line; the original carrier is placed on the ITU-T frequency grid. The square root of Allan variance of 10^-8 has been achieved for a period of 12 h.     

Frequency control of self-sustained pulsating laser diodes byuniform impurity doping into multiple-quantum-well structures

The self-sustained pulsating frequency in index guided AlGaAs multiple-quantum-well (MQW) laser diodes is controlled by impurity doping into the active region to reduce the relative intensity noise induced by optical feedback through a short optical path. Uniform n-type impurity doping into an MQW structure more effectively reduces the frequency by decreasing the differential gain than does modulation doping with an n-type impurity. Uniform doping of 1×10¹⁸ cm^-3 into each quantum well layer reduces the frequency to less than 0.8 GHz, which corresponds to half or two-thirds that of undoped lasers. The uniformly n-doped self-sustained pulsating lasers provided low noise characteristics with a relative intensity noise below 1×10³ Hz^-1 under an optical feedback of 20% even with a short optical path length of 60 mm     

1-Gsymbol/s 64-QAM Coherent Optical Transmission Over 150 km

Quadrature amplitude modulation (QAM) is an excellent modulation format for realizing optical communication systems with a high spectral efficiency of much greater than 1bit/s/Hz. We describe QAM coherent optical communication that we achieved by using heterodyne detection with a frequency-stabilized fiber laser and an optical phase-locked loop (OPLL) technique. The phase error variance of the intermediate frequency signal of the OPLL was 6.1times10^-3 rad. A 1-Gsymbol/s 64-QAM coherent signal was successfully transmitted over 150km     

The optically pumped rubidium maser

The optically pumped rubidium maser oscillator is the most recent addition to a growing number of atomic frequency standards. It is the first atomic frequency standard which is small enough and simple enough to be considered as a replacement for crystal oscillators. These factors and the extreme phase stability which results from the maser action make this device unique among all frequency standards. The device generates a microwave output at the ground-state hyperfine frequency of Rb⁸⁷(6835 Mc/s). The maser consists of a microwave cavity filled with Rb⁸⁷vapor and nitrogen gas. Oscillation occurs when the vapor is illuminated with filtered rubidium resonance radiation. The power output of the maser is 10^-10watts, and higher powers can be expected. In this paper the physical principles and construction of the device are described. The effects of optical pumping, buffer gas, and temperature on the maser are discussed, and experimental results are given. The short-term stability for observation times of about one second is expected to be about one part in 10¹². This may be increased by an order of magnitude by increasing the powser output to 10^-8watts. The long-term stability is expected to be comparable to that obtained in the passive rubidium standard (about one part in 10¹¹per month). These slow fluctuations arise from pressure shifts, light shifts, cavity pulling, and changes in the chemical composition of the buffer gas. The long-term stability can be improved by using the rubidium maser as the flywheel for an atomic beam frequency standard. Such a combination could be expected to have both long-term and short-term stabilities as great as one part in 10¹³.     

Superconducting-cavity stabilized oscillators with improved frequency stability

X-band microwave oscillators stabilized with superconducting niobium cavities with loaded Q's of about 10¹⁰achieved short-term frequency stabilities as low as σy=6 × 10^-16, and typical long-term fractional frequency drifts of ± 2 × 10^-13per day.     

Frequency instability measurement system of cryogenic maser oscillator

The frequency stability measurement of a new kind of secondary frequency standard, the whispering gallery mode maser oscillator, is reported. Based on a very simple design the beatnote comparison with a state-of-the-art cryogenic sapphire resonator oscillator gave a preliminary result of 10^-14 frequency instability at 1 s integration time. The measurement is limited by the microwave synthesis chain used to evaluate the maser stability.     

A New InP/InGaAs Double Heterojunction Bipolar Transistor With a Step-Graded InAlGaAs Collector Structure

An interesting InP/InGaAs double heterojunction bipolar transistor with a step-graded InAlGaAs layer at the base-collector (B-C) heterojunction is fabricated and studied. Simulated results reveal that the potential spike at the B-C heterointerface is completely eliminated. Experimentally, the operation regime is wider than 11 decades in magnitude of the collector current (I_c = 10^-12 A to I_c = 10^-1 A). Furthermore, the studied device exhibits a relatively high common-emitter breakdown voltage and low output conductance even at high temperature. In the microwave characteristics, the unity current gain cutoff frequency f_T = 72.7 GHz and the maximum oscillation frequency f _max = 50 GHz are achieved for a nonoptimized device (A_E = 6 times 6 mum²).     

High Extinction Ratio Mach–Zehnder Modulator Applied to a Highly Stable Optical Signal Generator

Research into optical modulators has made remarkable progress in recent years. This paper discusses the possibility of applying the high extinction ratio optical modulator to a high-stability and high-frequency (over 100 GHz) optical reference signal generator. High-frequency reference signals are generated by a highly stable optical two-tone generator, which is used for high-rate communication and astronomical application. One method to generate two optical signals is producing them from a pair of laser sources using an optical phase-locked loop for feed back control; however, the optical phase-locked loop has a stability problem in its operation. A good alternative method to the optical phase-locked scheme is the LiNbO³ Mach-Zehnder (MZ) optical intensity modulator, which is capable of generating two highly stable optical signals (upper sideband and lower sideband components) by applying a sinusoidal microwave signal to an input laser signal. The two optical signals require phase stability better than 10^-13 in the Allan standard deviation, vibration robustness, and polarization maintaining capability. The signal coherence loss estimated from the phase stability of the two optical signals generated by the MZ modulator shows that the optical MZ modulator has the ability to generate highly stable optical signals.     

Laser-cooled neutral atom frequency standards

The authors review the prospects for a frequency standard based on what is called the atomic fountain. Atomic fountain frequency standards have better signal-to-noise ratios than ion standards, but suffer from larger systematic effects. An analysis of a Cs microwave fountain predicts potential stability and accuracy improvements of a factor of 100 over other laboratory standards, with an ultimate accuracy near 10 ^-16. An optical frequency standard based on a two-photon transition in xenon is analyzed and shown to have potential stability 5 orders of magnitude better than conventional Cs standards, and a possible accuracy of 10^-18. Neutral atom frequency standards will have much better signal-to-noise ratios than ion standards, but suffer from larger systematic effects, making the two technologies complementary in many ways     

Crystal growth, frequency doubling, and infrared laser performanceof Yb3+:BaCaBO3F

Yb:BaCaBO₃F(Yb:BCBF) has been investigated as a new laser crystal with potential for self-frequency doubling, Yb³⁺ in BCBF exhibits a maximum absorption cross section at 912 nm of 1.1×10^-20 cm² with a bandwidth (FWHM) of 19 nm. The maximum emission cross section at 1034 nm is 1.3×10^-20 cm² with a transition bandwidth of 24 nm. The measured emission lifetime of Yb³⁺ is 1.17 ms. An Yb:BCBF laser has been demonstrated with a Ti:sapphire pump source, and a measured slope efficiency of 38% has been obtained for the fundamental laser output. Single crystal powders of BCBF have been compared with KD ⁺P for a relative measure of the second-harmonic generating potential, yielding d_eff(BCBF)~0.26 pm/V. The phasematching angle has been estimated from the refractive index data for type I second-harmonic generation of 0.517 μm light; the predicted angle is 37° from the c-axis. The growth, spectroscopy, laser performance, and linear and nonlinear optical properties of Yb:BCBF are reported