Mode hops

The Impedance Z can Increase to very high values. If this happens, the oscillator prefers to oscillate In resonance with an anharmonic frequency. Sometimes this condition Is met for only a short time and the oscillator oscillation jumps back and forth between a basic and an anharmonic oscillation or It remains as an anharmonic oscillation. This phenomenon Is well known as "mode hopping". In addition to the noise of the rate signal created, this may also lead to Incorrect termination of a coating because of the phase jump. It Is Important here that, nevertheless, the controller frequently continues to work under these conditions. Whether this has occurred can only be ascertained by noting that the coating thickness Is... 

INFICON has developed a new technology for overcoming these constraints on the active oscillator. The new system constantly analyzes Ihe response of the crystal fo an applied frequency not only fo determine the (series) resonance frequency, but also fo ensure that the quartz oscillates in the desired mode. The new system is insensitive te mode hopping and the resultant inaccuracy. It is fast and precise. The crystal frequency is determined 10 times a second w/ith an accuracy to less than 0.0005 Hz. 

Thus, perturbation measurements are best made when (1) highest accuracy is not necessary and (2) a substantial perturbation only to the region between IDTs is anticipated or (3) such a large perturbation is anticipated that it would be difficult to differentiate between a mode hop and the perturbation itself (e.g., the transition from a dry surface to one covered with liquid) or (4) such a large change in insertion loss is anticipated over the course of the measurement (e.g., a viscoelastic transition in a polymer layer) that it is not possible to set up and maintain a stable oscillator circuit. 

Figures 10.2 and 10.3 demonstrate the value of frequent calibration for the case of an unstable laser. An unstabilized single-mode diode laser exhibited mode hops over several hours, causing a laser frequency shift of 13 cm . If the Raman shift axis was not recalibrated after a mode hop, the apparent Raman shift also changes by 13 cm. Spectra before and after a mode hop are shown in Figures 10.2A and 10.2B, along with their difference. The mode hop caused a severe subtraction artifact due to the imprecise Raman shifts. Figure 10.3 shows spectra before and after the mode hop, but with the laser and...

Figure 10.2. Spectra of benzene obtained one hour apart from a spectrometer with an unstable diode laser. Raman shift axis was not recalibrated between A and B, causing a 13 cm shift due to a laser mode hop. Both spectra and their difference use the same intensity scale.

Jang L, Chao S, Holl M, Meldrum D (2007) Resonant mode-hopping micromixing. Sens Actuators A Phys 138(1) 179-186... 

Figure 4.18 Typical output spectrum from an ALGaAs diode Laser, as a function of injection current (Left) and temperature (rigiit) multi-mode operation at Low injection current and single-mode operation at liigh injection current are indicated in the insets. Mode-hop Locations are marked by the arrows...

Fig. 5.56a,b. Discontinuous tuning of lasers (a) part of the neon spectrum excited by a single-mode dye laser in a gas discharge with Doppler-limited resolution, which conceals the cavity mode hops of the laser (b) excitation of Na2 lines in a weakly collimated beam by a single-mode argon laser. In both cases the intracavity etalon was continuously tilted but the cavity length was kept constant... 

Fig. 5.66. (a) Axial resonator modes within the spectral gain profile (b) temperature tuning of the gain maximum and (c) mode hops of a quasi-continuously tunable cw PbS Te diode laser in a helium cryostat. The points correspond to the transmission maxima of an external Ge etalon with a free spectral range of 1.955 GHz [5.112]... 

A simple mechanical solution for wavelength tuning of the dye laser in Fig. 5.85 without mode hops has been realized by Littman [5.182] for a short... 

Fig. 5.89. Continuous mechanical tuning of the dye laser wavelength without mode hops by tilting mirror M2 around an axis through the intersection of two planes through the grating surface and the surface of mirror M2...

With such a system single-mode operation without etalons has been achieved. The wave number could be tuned over a range of 100 cm without mode hops. 

M. de Labachelerie, G. Passedat Mode-hop suppression of Littrow grating-tuned lasers. Appl. Opt. 32, 269 (1993)... 

To obtain generations in the regions of mode-hops for conventional FP lasers a simple design with a small external cavity was proposed. A glass plate mounted in front of the diode chip acts as the low-reflectivity mirror of an extended resonator and helps to select particular longitudinal modes within gain profile, even where laser action is not possible without the feedback. 

In 1999 blue DLs became commercially available. InGaN MQW DLs were demonstrated to have a lifetime of more than lOOOOh under conditions of room temperature cw operation. Singlemode emissions were produced in the region of 390-450 nm. Maximum output power is about 30 mW. With an external cavity the narrowband 1 MHz la.ser linewidth can be coarsely adjusted over a tuning range of 2.5 nm and then fine-tuned by a piezo stack over a more than 20 GHz range without mode hops. 

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