Servo loop

Commercial Ar+ lasers may incorporate a light regulator accessory (also called a servo loop stabilizer) which minimizes fluctuations in the laser output power thus improving long term power stability to better than 0.5% rms. 

The controlled variables cgpt that are released will identify the initial servo loops,... 

The feasible point will be attained by changing set-points of the servo loops while keeping cPpt and cpeg tight by the regulatory loops. 

The set points of the released constraints c p and m (if available) are the only free variables to adjust at each optimization step. Whenever new constraints become active at a search step, constraint sets are modified to define the regulatory and the servo loops. If there remains no search direction at a search point due to encountering new constraints, Kuhn-Tucker multipliers are reevaluated at that point. If they are all negative, an apparent minimal is found and the search is terminated otherwise, it is continued with the new search direction selected. 

Experimental Techniques. A block diagram of the experimental set-up used for saturated absorption experiments is shown in Figure 1. The argon laser is a commercial 4W tube in a home made cavity. This cavity is made of three Invar rods, decoupled from the tube in order to avoid vibrations. Line selection is made with a prism, and single frequency operation is obtained with a Michel son interferometer. The laser can be frequency locked to a stable Fabry-Perot resonator with a double servo-loop acting on a fast PZT for line narrowing and on a galvo-plate for wide tuna-bility. This results in a linewidth of less than 10 KHz and a continuous tunability of 6 GHz. 

EBIT as shown in Fig. 2. The trapped Si + ions will lie at the laser beam waist within the enhancement cavity, and to keep the finesse of the cavity as high as possible it is necessary for the high reflectivity mirrors to lie within the EBIT vacnnm chamber. The Ti sapphire laser will be locked to the high finesse cavity nsing the rf sideband locking techniqne [33]. Fast frequency fluctuations will be corrected using an acousto-optic modulator in a double-pass configuration, whilst the slower branch of the servo loop will use a piezo-mounted mirror in the laser cavity. 

Fig. 3. Block diagram of SPV apparatus using redox electrochemical liquid pickup. Light is conducted to the sample surface by fiber optics. A servo loop adjusts the light intensity to keep the SPV constant at all wavelengths.

Both of these are routine tasks when FM is employed. The relative magnitudes of the cavity background signals with- and without the sample present give T/To. Ql is determined from a scan, for which the source and cavity frequencies are maintained at the sample resonance whilst the FM deviation is scanned de-crementally from its upper value to near zero. The servo loop is unable to maintain the lock at zero deviation, hence the direction of scan. Examples of a series of scans for various values of the FM depth on the cavity at resonance frequency 175 GHz are shown in Figure 6.15. These cavity scans show a broad maximum whose value reflects the Q of the cavity. 

If the scene is moving (e.g. during deformation measurements or if a moving object is to be tracked) the first closed servo loop drives the line of sight only approximately to the centre... 

Pahk HJ, Lee DS, Park JH (2001) Ultra precision positioning system for servo motor-piezo actuator using the dual servo loop and digital filter implementation. Int J Mach Tools Manuf 41 51-63... 

All atomic clocks are based on the same servo-loop scheme (Figure 11.1). An internal atomic oscillator at cOai is used to lock an external or local atomic oscillator at frequency (Dq- The local oscillator is used to probe the atomic transition at (0, and... 

FIGU RE 11.1 A schematic diagram of an atomic clock. The atomic reference (0) 1) i provided by an atomic transition of an atom or a molecule. The fluorescence of the transition is detected by the oscillator (the local oscillator), which excites the atomic system at CBo. Before each interrogation, to make cOq a close as possible to cOac the servo loop adjusts the CBo fluorescence signal. A part of the signal of the local oscillator at cOo delivers the clock signal. 

Using a double servo loop for fast stabilization of the laser frequency onto the transmission peak of a Fabry-Perot Interferometer (FPI) and a slow loop to stabilize the FPI onto the first derivative of a forbidden narrow calcium transition, Barger et al. constructed an ultrastable cw dye laser with a short-term linewidth of approximately 800 Hz and a long-term drift of less than 2 kHz/h [218]. Stabilities of better than 1 Hz have also been realized [219, 220]. 

When the detector signal is fed to a lock-in amplifier that is tuned to the modulation frequency /i, the lock-in output can drive a servo loop to bring the phase difference cpQ back to zero. For (p((o) = 0, we obtain from (2.78,2.79)... 

Figure 1. Block diagram of an optical frequency divider showing two servo loops where the laser is locked to a reference cavity and the cavity to a radio frequency standard. The LiTa03 modulator is driven at

BS to a detector (Fig. 5.44a). The detector output Vo is compared with a reference voltage Vr and the difference A V = Vd — is amplified and fed to the power supply of the laser, where it controls the discharge current. The servo loop is effective in a range where the laser intensity increases with increasing current. 

Fig. 5.52. Long-term stabilization of the laser wavelength locked to a reference FPI that in turn is locked by a digital servo loop to a molecular transition...

The actual stability obtained for a single-mode laser depends on the laser system, on the quality of the electronic servo loop, and on the design of the resonator and mirror mounts. With moderate efforts, a frequency stability of about 1 MHz can be achieved, while extreme precautions and sophisticated equipment allow a stability of better than 1 Hz to be achieved for some laser types [5.81]. 

---