We investigate a new phenomenon, where a reciprocal fiber ring laser switches from bidirectional to unidirectional operation above a certain pump power threshold. We present significant simplifications regarding earlier experiments, which for the first time allow the identification of individual nonlinear effects. We highlight the unique role of stimulated Raman scattering in triggering unidirectional operation.
We recently reported on a new form of unidirectional lasing in a fully reciprocal kilometre long fiber ring cavity. It is a threshold like phenomenon where unidirectionality is triggered at full laser operation far above the laser threshold. Directionality is the consequence of asymmetric round trip loss, which arises due to differing nonlinear loss because of Raman scattering for the two directions. Asymmetry can be altered by seeding any direction at Stokes wavelengths to define the final direction of the laser. This was achieved by a simple straight cleaves at WDM outputs, used to tailor the excited spectrum.
We report on the observation of a new phenomenon, occurring in a fiber ring laser. This phenomenon describes the transition from an initially bidirectional emission of a reciprocal fiber ring laser to a unidirectional emission at a certain pump power threshold. In addition, the final direction is not predefined but appears to be randomly chosen every time the threshold is exceeded. Therefore, we term this new phenomenon as direction instability. In addition we provide a first discussion of how the pump power threshold and the final direction can be influenced by the length and the loss of the cavity.
We demonstrate a new technique to generate a continuous-wave supercontinuum based on the stimulated Raman effect in an Yb-based ring laser configuration. Continuously pumping this ring cavity with a maximum of 19W optical power, we were able to excite up to 6 Stokes orders and achieved wavelengths up to 1600nm. Due to the feedback mechanism of the ring cavity the generated spectrum does not exhibit plain and separated stokes peaks but the fundamental Raman nature of the spectrum is altered additional nonlinear effects. This results in a dense coverage and an almost complete excitation of the wavelength range from the laser wavelength to the highest stokes wavelength and hence in a continuous-wave supercontinuum. Since the main mechanism of broadening is the Raman effect, we do not rely on anomalous dispersion and modulation instability as typically required to seed continuous wave supercontinuum generation.
Various applications in optics and photonics employ a Tunable Focus Lens (TFL) to obtain a minimum beam spot of a laser beam at different locations along the direction of beam propagation. Using a TFL to achieve a minimum beam spot size at different planes is critical for several optical imaging and sensing applications. As focal length of TFLs is generally controlled through the amplitude of an input voltage or the current signal, the response time of a TFL-based sensor or imaging system depends on the time required produces a minimum beam spot in the observation plane. Therefore, the system response time or sampling rate depends on the number of voltage/current samples to ascertain a correct focal length value which yields a minimum beam spot. Due to a partial or a total lack of knowledge of the ideal voltage/current value that would produce a minimum beam spot, starting at a corner or a random voltage/current value and incrementally increasing or reducing it would be highly inefficient as converging to a minimum spot might require several steps. We propose an efficient method which results in a significant reduction in the number of voltage/current steps and experimentally validate our claims.
This paper presents a data transfer scheme using multi-focus tunable lenses. The design involves the use of a standard laser source and a variable focus agile lens to steer to the laser beam that passes through the lens. In our proposed system, the beam steer angle depends on an input electrical signal which drives the tunable lens. Therefore the beam steer angle is made to follow the variations in the input electrical drive signal. This is extremely interesting for data transfer applications as the data signal can be used as the input drive signal to the lens. The laser beam is steered according to the input data voltage levels and when the beam is incident on a photo-detector of a finite size, only a fraction of its total incident optical power is received by the photo-detector. This power contribution is proportional to the fraction of the total number of photons per unit area which are incident on the active area of the detector. The remaining photons which are not incident on the photo-detector do not contribute to the received power at the photo-detector. We present the theory of beam steering through a tunable lens and present a theoretical framework which governs data transfer through the proposed method. We also present the transfer function of the proposed system which helps us to calculate its essential theoretical performance parameters such as modulation depth and bit error rates. We also present experimental results to demonstrate efficient data transfer through the proposed method. As tunable lenses are primarily deployed in motion-free multi-focus cameras hence most of the modern portable devices such as cellphones and tablets use these lenses to operate the in-built variable focus cameras that are part of these devices. Because tunable lenses are commonly present in several different portable devices, the proposed method of data transfer between two devices is highly promising as it expands the use of the already deployed tunable lenses with minimal changes to the fundamental architecture or operation of any portable device.
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