Microwave signal generation with injection-locked laser diodes

Heterodyne detection of the light from two slave lasers injection locked to FM sidebands of a modulated master laser is used to generate a narrowband microwave signal at 10.5 GHz.     

Microwave signals generated by optical heterodyne betweeninjection-locked semiconductor lasers

Single-mode-laser rate equations with added Langevin noise sources are used to study injection-locked semiconductor lasers. Two slave lasers are frequency-locked on the same or different sidebands of a current-modulated master laser. The optical heterodyne between the two secondary lasers is characterized. It is demonstrated that the frequency stability of the source modulating the master laser is preserved on the sidebands and partially transferred to the slaves. A linear model is first investigated. Static operation conditions and small-signal behavior are then calculated. Direct simulation of the rate equations for each laser is next achieved. This highlights the validity domain and limitations of the linear model. A more complete set of results-such as laser and heterodyne spectra-is also obtained. It is moreover shown that synchronization of the slave laser diodes by optical injection-locking leads to strongly correlated, while not identical, laser fields. Finally, simulation results are compared to experimental data     

Theory of FM laser oscillation

The paper presents a detailed analysis of FM laser oscillation which includes the effect of arbitrary atomic lineshape, saturation, and mode pulling. Such oscillation is achieved by means of an intracavity phase perturbation, and is a parametric oscillation wherein the laser modes oscillate with FM phases and nearly Bessel function amplitudes. One principal idea is that of the competition between different FM oscillations. The effect of the intracavity phase perturbation is to associate a set of sidebands with each of the previously free-running laser modes. While the free-running laser modes experienced their gain from essentially independent atomic populations, the competing FM oscillations to a large extent see the same atomic population; and in cases of interest the strongest of these oscillations is able to quench the competing weaker oscillations and establish the desired steady state condition. Results of the analysis include the following: threshold and power output, amplitudes and phases of all sidebands, frequency pulling of the entire oscillation, time domain behavior, distortion, super-mode conversion efficiency, and effect of mirror motion. Results of numerical application of the theory to a number of specific cases are given.     

Experimental relative frequency stabilization of a set of lasersusing optical phase-locked loops

A relative frequency stabilization technique using optical phase locking of miniature diode pumped Nd:YAG ring lasers is described. The master laser is RF phase modulated with a modulation index up to 7.4, and slave lasers are locked up to 21 master laser sidebands with a frequency stability better than 3 kHz     

Higher order FM mode locking for pulse-repetition-rate enhancementin actively mode-locked lasers: theory and experiment

We present a novel higher order FM mode-locked technique for active mode-locked lasers which utilizes the higher order sidebands generated by an intracavity phase modulator to establish the mode locking. The resulting mode-locked output exhibits an enhancement of the pulse repetition rate over the modulation frequency by an integral multiple. The higher order FM mode locking is studied theoretically in a laser with a homogeneous gain medium, and simple analytical expressions are obtained to characterize the output pulses. It is shown that the scheme not only enhances the pulse repetition rate but also ensures chirp-free pulses and is effective in eliminating the output pulse phase-state instability, which are commonly observed in conventional FM mode-locked lasers. The effect of group velocity dispersion and cavity nonlinearity is also investigated through numerical solution of the self-consistency equation applied to a fiber ring laser. Finally, detailed experimental results on repetition rate enhancement in fiber lasers are presented and shown to be in good agreement with the theoretical results     

Optical frequency modulation and intensity modulation suppression in a master-slave semiconductor laser system with direct modulation of the master laser

An injection-locked laser system where the master laser is directly modulated is analyzed using rate equations. Both quasi-static and dynamic analyses are carried out in order to ensure that the parameters lie inside the locking range. The analysis is valid for all injection levels. The quasi-static analysis provides a good basis for explaining the phenomena. It is shown that, for a given detuning frequency, maximum suppression of intensity modulation (IM) occurs at a specific value of the injection ratio. At low frequencies, the frequency modulation (FM) index of the slave laser bears a constant ratio to the FM index of the master laser of less than unity. It is illustrated that the direct FM scheme is only viable for modulation frequencies up to about 100 MHz. Large IM suppression can only be achieved for large values of the linewidth enhancement factor of the slave laser, small magnitude of the detuning frequency, and low injection ratio. The latter two conditions are associated with narrow limits on stable operation, and care should be taken to avoid instability.     

Coherent FSK transmitter using a negative feedback stabilised semiconductor laser

An electrical negative frequency feedback scheme is applied to a semiconductor laser to improve its direct frequency modulation performance in an FSK transmitter. An error signal caused by imperfect FM response is extracted through heterodyne discrimination detection with a stable master laser and fed back to the FM semiconductor laser.     

Small signal analysis of optical FM signal amplification by aninjection-locked type semiconductor laser amplifier

A small-signal analysis for optical FM signal amplification by an injection-locked type semiconductor laser amplifier with consideration of the spurious intensity modulation (SIM) is presented. In general, the performance of optical FM signal amplification by a laser amplifier is affected by the value of the injection power, by the performance of the injecting laser (the master laser), and by the difference between the emission frequencies of the master and slave optical cavities. In particular, when the modulation frequency is very low, the output frequency deviation (FD) of the laser amplifier turns out to coincide with that of the master laser. However, SIM of the laser amplifier in this case is influenced by the injection conditions. A suitable definition, the ratio of the output FD to SIM to the input FD to SIM, is given to evaluate the performance of an injection-locked-type semiconductor laser amplifier     

Optical FM Signal Amplification by Injection Locked and Resonant Type Semiconductor Laser Amplifiers

Optical FM signal amplification by semiconductor lasers is studied by emphasizing their bandwidth characteristics. The laser is operated either in an injection-locked mode or in a resonant amplification mode by keeping the drive current above or just below its threshold. The bandwidths of both amplifiers are evaluated by the reduction in modulation sidebands and are compared with the bandwidths measured statically by scanning the frequency of incident CW wave. The radic G = 25 GHz gain bandwidth product is obtained for both operation modes using a double heterostructure AlGaAs semiconductor laser. The bandwidth obtained in the above procedure is in good agreement with theoretical results.     

FM noise suppression and linewidth reduction in an injection-locked semiconductor laser

An expression is derived for the FM noise spectrum of an injection-locked semiconductor laser. For increasing injection level the FM noise spectrum and the lineshape change gradually from those of the slave laser to those of the master laser. The ultimate linewidth reduction is obtained if the locking bandwidth is large enough to accommodate the frequency fluctuations and if the detuning is controlled carefully.     

Phase-sensitive interrogation of fiber Bragg grating resonators for sensing applications

This paper discusses a phase-sensitive technique for remote interrogation of passive Bragg grating Fabry-Pe/spl acute/rot resonators. It is based on Pound-Drever-Hall (PDH) laser frequency locking, using radio-frequency phase modulation sidebands to derive an error signal from the complex optical response, near resonance, of a Fabry-Pe/spl acute/rot interferometer. We examine how modulation frequency and resonance bandwidth affect this error signal. Experimental results are presented that demonstrate, when the laser is locked, this method detects differential phase shifts in the optical carrier relative to its sidebands, due to minute fiber optical path displacements.     

Experiments on a semiconductor laser pumped rubidium atomic clock

Experiments were carried out to evaluate the performances of a semiconductor laser pumped rubidium (⁸⁷Rb) atomic clock. Two kinds of Rb gas cells were used and their performances were compared [gas cell A (natural rubidium (⁸⁷Rb/⁸⁵Rb =frac{3}{7}) and buffer gases) and gas cell B (⁸⁷Rb and buffer gases)]. The highest microwave frequency stabilities were estimated as3.4 times 10^{-12} tau^{-1/2}and2.7 times 10^{-12} tau^{-1/2}at the optimal gas cell temperatures of 60°C and 48°C for the gas cellsAandB, respectively (τ: integration time). The light shift, i.e., microwave frequency shift induced by laser light, was measured as -0.50 Hz/MHz and -0.11 Hz/MHz for the gas cellsAandBat their optimal operating conditions given above. As an improved experiment by utilizing high temporal coherence of the laser, a novel double resonance spectral line shape with a drastically narrower linewidth was demonstrated. A technique, similar to FM laser spectroscopy, was employed for this purpose by utilizing laser FM sidebands which are induced by microwave frequency modulation and nonlinear susceptibility of three-level⁸⁷Rb atoms. The minimum linewidth obtained was 20 Hz, which can be used as a sensitive frequency discriminator for an improved⁸⁷Rb atomic clock.     

Comparison of Indirect Optical Injection-Locking Techniques of Multiple X-Band Oscillators

Experimental results of indirect optical injection-locking of two X-band FET oscillators are presented. An S-band master source is used to synchronize both oscillators simultaneously, with 18-MHz locking range using the fiber-optic link nonlinearity. The source of the optical link nonlinearity is traced to the laser diode by interferometric measurement. Both the laser diode and the FET oscillator nonlinearities can be exploited to achieve frequency multiplication of the master oscillator signal. The merits of these different methods are evaluated based on the locking range and the FM noise level of the injection-locked oscillator.     

原子干涉仪中拉曼激光的相位-频率双调制稳频

为降低拉曼激光的频率噪声,提出了一种相位-频率双调制稳频技术。用光纤电光相位调制器对激光进行调制并产生大失谐边带;用射频信号对光纤相位调制器的微波驱动信号进行频率调制,通过锁相放大法将一个大失谐边带锁到铷原子的饱和吸收谱线上。利用该技术实现了拉曼种子激光的稳频和2 GHz的移频,拉曼激光的线宽大幅压窄到56 kHz,预期拉曼激光频率噪声引起的原子干涉重力仪的单次测量噪声可降低到7×10-9/s2。     

Impact of Sampling Clock Phase Noise on Σ∆ Frequency Discriminators

SigmaDelta frequency discriminators (SigmaDeltaFDs) convert instantaneous frequency deviations of a carrier signal to digital. They are used for decoding narrowband phase or frequency modulated signals in communication receivers, self calibration of RF frequency synthesizers and in digital phase locked loops. In this paper, the impact of reference (sampling) clock phase noise on a SigmaDeltaFD's spurious-free dynamic range (SFDR) is derived. It is shown that for SigmaDeltaFDs with jittered sampling clock, in addition to FM sidebands, a high baseband tonal content is generated degrading overall SFDR. The reference clock phase noise impact is derived mathematically, and two commonly used SigmaDeltaFDs circuits are designed and implemented to verify the results experimentally. Experimental results are shown to match the theoretical prediction of SFDR within 3 dB.     

FM mode-locked high-pressure CW RF-excited CO2 waveguidelaser

The authors report the results from a study of a FM mode-locked continuous-wave (CW) RF-excited CO₂ waveguide laser operated at 0.25-2 atm gas pressures. It is shown that electrooptic FM modulations can be efficiently used to mode lock a CW CO₂ laser. The combination of a high gas pressure and a high modulation frequency makes it possible to generate pulses which are substantially shorter than those previously reported for CW mode-locked CO₂ lasers. A theoretical approach is used for simulation of the FM mode-locked laser. The experimental pulses of a few hundred picoseconds FWHM are considerably shorter than previously reported for CW mode-locked CO₂ lasers. The experimental results are compared with the results of numerical calculations using a frequency domain simulation model     

Analyses of numerical and experimental results for optical signal generation using heterodyne scheme

In this paper, we have proposed a heterodyne technique to generate an optical millimeter-wave signal for ultra-wideband communication. First, we have investigated the characteristics of semiconductor lasers locked to another semiconductor under the RF modulation having many sidebands. The RF-modulated master laser is represented by a series of Bessel functions. This model is then inserted into Lang's rate equations. By numerically solving the resulting rate equations, we have determined the locked laser output characteristics as well as the RF spectrum of the beat signals. The result is that the unselected sidebands can produce undesired beat signals whose power may be comparable to that of the desired beat signal. Furthermore, their strength is affected by the injected ML light power. With reduced ML light, undesired beat signals and the injection-locking bandwidth can be suppressed. Second, we have experimented a new technique for generating millimeter-wave signals from a semiconductor laser. A 32 GHz signal is generated using a multisection semiconductor laser operated under a continuous wave by injecting optical pulses at a repetition rate equal to the fourth subharmonic (8 GHz). The generated millimeter-wave signal exhibits a large subharmonic suppression ratio, a large frequency detuning range, low levels of phase-noise and a large locking range. These simulation results are confirmed by experimental results. The high-frequency signal can be used in the field of ultra-wideband communication employing local multipoint distribution system (LMDS), wireless local loop (WLL) and mobile broadband system (MBS).     

The effects of small contaminating signals in nonlinear elements used in frequency synthesis and conversion

Many electronic systems use nonlinear elements to add or subtract two frequencies or to multiply or divide a frequency by an integer. Some level of contamination by small undesired signals is always present and the ability to predict the effects produced by their passage through the nonlinear elements is important in analyzing system performance. These effects can often be predicted, for frequency mixing (addition and subtraction), multiplication and division, by decomposition of the contaminating signal into equivalent AM and FM sidebands whose effects are more easily estimated. One important effect that occurs in frequency division is a sampling process which translates the frequencies of the interfering signals. A method for predicting these effects is explained and experimental results, demonstrating the application and applicability of the method, are reported.     

基于级联调制器和四波混频效应的24倍频微波信号的光学生成

提出了一种基于级联马赫-曾德尔调制器 (MZM)和半导体光放大器(SOA)的24倍频微波信号 光学生成方案,具有覆盖频段高、杂波抑制效果好等优点。在本方案中,低频微波信号分别 通过两个级联 MZM对连续光源进行调制,调节直流偏置使两个MZM均工作在最大偏置点,以抑制奇数阶光分 量;进一 步调节两个MZM的调制深度,并结合可调谐电相移器(TEPS)和可调谐光相移器(TOPS)引入相 移,完全抑 制第2个MZM输出的±2阶光分量和光载波,得到±4阶光分量;再经过SOA发生四波混频(FWM )效应,形成±12 阶光分量;滤波后拍频可以获得24倍频微波信号。最后,搭建了实验和仿真系统,分别以11．0GHz,11.5GHz和12.0GHz的微波信号为驱动,得到间隔为264、276GH z和288GHz的±12阶光边带,有效验证了方案的可行性。     

Direct Frequency Modulation In AlGaAs Semiconductor Lasers

Direct frequency modulation characteristics in three different AlGaAs lasers - a channeled-substrate planar (CSP) laser, a buried-heterostructure (BH) laser, and a transverse-junction-stripe (TJS) laser- are studied theoretically and experimentally. Experimental FM responses are measured by using the Fabry-Perot interferometer and birefringent optical filters in the 0-5.2 GHz modulation frequency region. Experimental FM response dependence on modulation frequency, dc bias level, and stripe structure are successfully explained by the theoretical analyses considering both the carrier density modulation effect and the temperature change effect. FM response in the low modulation frequency region from 0 to 10 MHz, gradually decreasing with the modulation frequency, stems from the thermal effect. FM response in the high modulation frequency region from 10 MHz to 5.2 GHz is caused by the carrier effect. A flat FM response of several hundred MHz per 1 mA is observed in the CSP and TJS lasers, but a V-shaped FM response is obtained in the BH laser. Resonance peak due to relaxation oscillation and cutoff characteristics are observed in several gigahertz regions. Weak lateral mode confinement, strong vertical mode confinement, carrier injection outside the effective core region, and p-side down mounts are effective ways to achieve a flat and efficient FM response with a small spurious intensity modulation.