Font Size: a A A

Analysis And Control Of Vibration And Residual Amplitude Modulation In Laser Frequency Stabilization

Posted on:2016-04-17Degree:DoctorType:Dissertation
Country:ChinaCandidate:H ShenFull Text:PDF
GTID:1228330464460418Subject:Atomic and Molecular Physics
Abstract/Summary:PDF Full Text Request
Ultra-stable oscillators at optical frequencies, due to their high spectral purity and frequency stability, have been widely used in a variety of research fields such as optical frequency standards, high-precision spectroscopy, interferometric gravitational-wave detection, and the generation of ultra-stable microwave. Locking the lasers on the resonances of the high-finesse cavity to improve the frequency stability, Pound-Drever-Hall (PDH) frequency locking technique is one of effective approaches to obtain narrow-linewidth stable lasers. The ambient vibration and residual amplitude modulation (RAM) have the potential to be two of the major sources affecting the laser frequency stability in the PDH frequency stabilization. To improve the frequency stability, this thesis focuses on the exploration and research of active vibration isolation, quantitative analyses of RAM, and active RAM control.To lay a foundation for the design and optimization of control loop in the subsequent laser frequency stabilization system, firstly the open-loop response of the operational amplifier is measured and analyzed in detail. As the important components of servos in feedback systems such as laser frequency stabilization, active vibration and RAM control, the open-loop responses of operational amplifiers have a great impact on their stability and related performance. In this thesis the open-loop response of an operational amplifier (LF356) is measured and analyzed using three methods, and the open-loop DC gain and 3-dB bandwidth are obtained. The measurement and the accompanying analyses indicate that in certain circumstances the closed-loop response is indeed restricted by the open-loop response. Consequently it is necessary to consider the open-loop responses to achieve realistic characteristics in the design and optimization of circuits with high gain and wide bandwidth.To isolate the measuring system from the ambient vibration, a compact system for active vibration isolation based on a PZT and a piezoelectric accelerometer is investigated. The mechanical resonances are suppressed effectively via electrical damping instead of mechanical damping, a feature improves the high frequency response. Using a displacement-sensing strain gauge attached to the PZT, an auxiliary control loop eliminates the drift associated with a large servo gain at DC and ensures a stable control point. Following these approaches, the vibration isolation between 1 Hz and 200 Hz is achieved and the isolation reaches its maximum of 60 times near the vibration frequency of 20 Hz. For extending the isolation to lower vibration frequencies, a scheme for active vibration control using a voice coil as an actuator is implemented and discussed. After active control, the vibration at 1-20 Hz is reduced to 1×10-7 g/Hz1/2, and a bandwidth of 2 Hz-600 Hz and a rejection peak of-100 at 20 Hz are achieved. The noise sources and possible improvement are also analyzed in detail with the help of constructing the entire loop response and a noise model.We theoretically derivate the PDH error signal with RAM, and analyze the quantitative model of RAM induced by the birefringence of the electro-optic (EO) crystal and parasitic etalon effect. Based on the birefringence effect without the parasitic etalon, the model of RAM in the presence of the optical cavity is added a constant related to the contrast of the cavity, and the representation of RAM is maintained. An improvement of the contrast is beneficial to degrade RAM. Moreover, the influence on the PDH error signal and locking offset are analyzed systemically, when the parasitic etalons locate in several typical places of the optical path. The character of RAM is also discussed by numerical calculation in consideration of the etalon from the coupling between the cavity and its incident or output surface.An active control scheme of RAM using a separate sensing path is implemented experimentally and analyzed in detail. In this scheme, an added EO crystal inserted in front of EOM is employed to improve high-frequency response and a slow temperature control loop with a large dynamic range and high low-frequency gain is also adopted for a stable and wide-bandwidth loop. With active control, RAM is suppressed to below 1×10-6 at averaging times of 0.02-1000 s, reaching a minimum of 2×10-7 at ~2 s. The corresponding RAM-induced frequency instability is below the thermal noise limit of 1×10-15 (0.02-1000 s). By comparing the two sets of rejection ratio, one inferred from the out-of-loop measurement of RAM with laser unlocked and the other observed in the optical heterodyne beat between a perturbed laser and a stable reference, it is experimentally verified that the presence of the optical cavity does not change the representation of RAM induced by the birefringence and the effectiveness of RAM control is consistent in the sensing/control path and PDH locking path.
Keywords/Search Tags:optical frequency standard, optical cavity, laser frequency stabilization, open-loop response, active vibration isolation, mechanical resonance, electric damping, residual amplitude modulation, parasitic etalon
PDF Full Text Request
Related items