Laser frequency stabilisation in the telecommunications band was realised using the Pound-Drever-Hall (PDH) error signal. The transmission spectrum of the Fabry-Perot cavity was used as opposed to the traditionally used reflected spectrum. A comparison was done using an analogue as well as a digitally implemented system. This study forms part of an initial step towards developing a portable optical time and frequency standard.
The frequency discriminator used in the experimental setup was a fibre-based Fabry-Perot etalon. The phase sensitive system made use of the optical heterodyne technique to detect changes in the phase of the system. A lock-in amplifier was used to filter and mix the input signals to generate the error signal. This error signal may then be used to generate a control signal via a PID controller. An error signal was realised at a wavelength of 1556 nm which correlates to an optical frequency of 1.926 THz. An implementation of the analogue PDH technique yielded an error signal with a bandwidth of 6.134 GHz, while a digital implementation yielded a bandwidth of 5.774 GHz.
Air/silica Microstructured Optical Fibers (MOFs) offer new prospects for fiber based sensor devices. In this paper, two
topics of particular significance for gas sensing using air guiding Photonic Bandgap Fibers (PBGFs) are discussed. First,
we address the issue of controlling the modal properties of PBGFs and demonstrate a single mode, polarization
maintaining air guiding PBGF. Secondly, we present recent improvements of a femtosecond laser machining technique
for fabricating fluidic channels in PBGFs, which allowed us to achieve cells with multiple side access channels and low
We present results obtained from the first all-fiber, lensless, optical correlation spectroscopy gas sensor for acetylene
(C2H2). In the reported sensing configuration, hollow-core photonic bandgap fiber (PBGF) is employed to contain all gas
samples required for optical absorption measurements. This sensor relies upon comparison of the absorption spectrum of
acetylene held in a 'reference gas cell' to that of a gas sample under test, which is contained in the 'measurement gas
cell'. Ingress of the test gas mixture into the measurement cell is achieved via femtosecond laser-machined micro-channels
running from the surface of the PBGF to its hollow core. Stable, lensless optical interrogation of the
measurement cell is guaranteed by means of arc fusion splices to standard (solid-core) single-mode fiber (SMF). The
reference cell is filled with acetylene at atmospheric pressure, and is permanently sealed at both ends by splices to SMF.
Therefore, being constructed entirely from optical fiber, both the reference and measurement gas cells are inherently
compact and coilable, and dispense with the need for lenses or other free-space optics for connection to the correlation
spectroscopy system. We quantify the acetylene concentration of various test gas mixtures and compare our sensor's
measured results with computer simulations.
Microstructured fibers (MOFs) are among the most innovative developments in optical fiber technology in recent years. These fibers contain arrays of tiny air holes that run along their length and define the waveguiding properties. Optical confinement and guidance in MOFs can be obtained either through modified total internal reflection, or photonic bandgap effects; correspondingly, they are classified into index-guiding Holey Fibers (HFs) and Photonic Bandgap Fibers (PBGFs). MOFs offer great flexibility in terms of fiber design and, by virtue of the large refractive index contrast between glass/air and the possibility to make wavelength-scale features, offer a range of unique properties. In this paper we review the current status of air/silica MOF design and fabrication and discuss the attractions of this technology within the field of sensors, including prospects for further development. We focus on two primary areas, which we believe to be of particular significance. Firstly, we discuss the use of fibers offering large evanescent fields, or, alternatively, guidance in an air core, to provide long interaction lengths for detection of trace chemicals in gas or liquid samples; an improved fibre design is presented and prospects for practical implementation in sensor systems are also analysed. Secondly, we discuss the application of photonic bandgap fibre technology for obtaining fibres operating beyond silica's transparency window, and in particular in the 3μm wavelength region.
We propose the use of optical fiber Bragg gratings in a non-invasive blood pressure waveform monitor. Bragg gratings can be written in a Fabry-Perot interferometric configuration to yield a method of strain measurement that has both a high resolution and a wide unambiguous range. This fiber Bragg grating Fabry-Perot interferometer (FBGI) can be used as a sensor to detect strain resulting from blood pressure applied to the walls of an artery situated near the patient’s skin. Strain measurements taken on the skin surface, typically over the radial artery at the wrist, are encoded as phase shifts of the FBGI signal. These phase shifts may be obtained by the analytic representation of the interferometer signal in the wavelength domain or by Fourier analysis in the frequency domain. For the proof of concept a realistic physical model was constructed to simulate pressure conditions at the actual sensor location. The operation of the device is demonstrated by measurements of pressure-pulse waveforms obtained in real-time. This sensor was also successfully tested on human patients, and these results are also presented. Since it yields continuous readings of blood pressure non-invasively, further application of the optical manometer may yield an alternative to conventional sphygmomanometry.
Most fiber optic interferometric refractometers suffer from large temperature sensitivities, especially those that employ long-period gratings. This paper presents results on a compound-cavity Michelson interferometer whose phase shift is only dependent on the refractive index of the analyte surrounding the fiber probe. This single probe Michelson interferometer uses mode coupling in a long-period grating to establish the two optical paths in a single fiber, and therefore presents a compact sensor for measuring the refractive index and other related properties (such as chemical concentration and composition) of various types of substances. It can also be used to measure the level of liquid in a container. Experimentally, we measured a phase change of 1.374 rad for a change in refractive index of 0.0132. The temperature sensitivity of one such compensated device is only -0.01 rad/°C, which is less than the temperature gradient of the refractive index of solutions of glycerine in water.
Fiber optic interferometric refractometers usually possess large temperature sensitivities, especially those based on individual long-period gratings. This work introduces a compound-cavity Michelson interferometer that uses mode coupling in a long-period grating to establish the two optical paths in a single fiber, and therefore presents a compact sensor for in vivo refractive index measurement. Successful operation of the athermal refractometer was demonstrated experimentally by comparing its phase shift due to temperature-dependent changes of the refractive index of the analyte with refractive index readings from a temperature-controlled Abbe refractometer.