Intracavity Phase Interferometry (IPI) has evolved into a powerful technique for high-precision metrology. In this method a parameter to be measured imparts a phase shift onto a pulse circulating in the laser cavity. The phase shift becomes a frequency shift applied to the corresponding frequency comb, which is measured by interfering the shifted comb with a reference comb created in the same laser cavity. Although fiber lasers are ideal for implementation of IPI, there are several challenges to achieve a deadband-free fiber IPI sensor. The huge amount of phase coupling between crossing pulses eliminates the small signal response. A successful observation of a beat signal was achieved in a polarization maintaining mode-locked fiber laser operating with orthogonally polarized pulses.
We present methods to enhance the response and reduce the noise in a broad class of ultrafast laser sensors, including but not limited to gyroscopes, accelerators, field sensors, and displacements.. Sensitivity enhancement through “exceptional points” was inspired by quantum mechanics. It is shown that this enhancement comes at the cost of a faster increasing noise. However, we demonstrate enhancement without noise increase in mode-locked lasers where two uncoupled pulses circulate at the same group velocity, without any coupling. Such sensors having reached the classical noise limit, quantum mechanical squeezing methods to further increase the signal to noise will be discussed.
For space missions, there is a need for fiber lasers of minimum power consumption involving stabilized frequency combs. We exploit the extreme sensitivity of the polarization state of circularly polarized light sent through polarization maintaining (PM) fibers to power and temperature variations. Low power nonlinear transmission is demonstrated by terminating a PM fiber by an appropriately oriented polarizer. The strong correlation between the power sensitivity of the polarization state and the temperature dependence of the birefringence of the PM fiber can be exploited for stabilization the optical length in fiber lasers and interferometers.
Temperature measurement of an optical fiber core is a challenge as there is no way to reach the tiny core in order to probe its temperature using regular methods. By exploiting the birefringence properties of a polarization maintaining (PM) fiber, a highly sensitive temperature sensor can be realized to monitor the variations of temperature in the optical fibers. This temperature sensor is particularly useful to detect very small temperature changes of the gain fiber in radiation-balanced fiber lasers. The sensor performance is demonstrated by measuring the temperature changes of a PM fiber attached to a Peltier cooler with the sensitivity of 0.02 C for 6 cm of PM fiber. Also, the temperature rise in the core of a piece of PM fiber carrying a few mW of a cw laser is measured to be 0.18 C mW with a response time of 125 microseconds. PM fibers are designed in different ways to create different indices of refraction along two orthogonal slow and fast axes. The polarization of a linearly polarized light input to a PM fiber along one of these axes is maintained while propagating in the fiber. Any other input polarization will be periodically modified along the PM fiber. The transmitted polarization state depends on the initial polarization, fiber length, and the birefringence of the fiber which is varied by small temperature changes. In the case of using single-mode gain fiber in radiation-balanced fiber lasers, the temperature detection can be done by attaching the gain fiber to the PM fiber sensor. The possibility of direct fiber core temperature monitoring in laser cooled fibers will be achieved by using polarization maintaining fiber for laser cooling. The same fiber can then be used as temperature sensor of its core using circularly polarized light as probe.
Intracavity phase interferometry (IPI) is a highly sensitive technique, using mode-locked lasers with two counter-propagating pulses, to measure small displacements, linear and nonlinear refractive indices, magnetic field, scattering, rotation and acceleration. Inertial sensors are needed for high accuracy navigation. Although gyroscopic response was successfully realized in discrete element mode-locked lasers with two circulating pulses, it is impractical in commercial systems. Fiber lasers are the most promising systems to implement IPI, owing to the possibility of producing ultrashort pulses with a compact and low-cost design. There are however substantial challenges to transfer the results of discrete optics lasers to fibers. Most publications on bidirectional fiber lasers report a different average group velocity for each of the circulating pulses, a situation rendering IPI impossible. This problem was solved by constructing an all-PM bidirectional mode-locked fiber laser with two portions of Er-doped fibers pumped through two WDMs to eliminate all the asymmetries in the cavity. In addition to ensuring equal group velocity for each pulse, the symmetric operation reduces the bias beat note. A fine control of the bias is obtained by tuning the pump powers of the two gain sections, in order to minimize the difference between the nonlinear phase accumulated in each direction between absorber and output coupler. Another challenge is to minimize the dead band created by the large scattering of the carbon nanotube. The solution that we implement is to force one of the intracavity pulses to propagate along the fast axis, the other along the slow axis.
Frequency combs are revolutionizing metrology, providing a link between optical and RF fre-
quencies. The cavity of a mode-locked laser determines the wavelength of a particular comb
tooth, and the teeth spacing. It is demonstrated experimentally that the average tooth
spacing of a frequency comb can be tuned by tilting an etalon inserted in the cavity. This pro-
perty amounts to a control of the average group velocity of a pulse circulating in the resonator.
We have shown that a mode-locked laser in which two pulses circulate constitutes a
sensitive phase sensor. This is because any phase shift between the two pulses is converted
into a frequency shift, equal to the ratio of the phase shift to the cavity round-trip time at the phase
velocity. The two frequency combs issued from the laser are split in frequency, a split measured
as a beat note on a detector recording the interfering combs. A laser gyro is an example of such
an Intracavity Phase Interferometer. A structure with periodic discrete resonances matching
several teeth of the comb will, because of the dispersion associated with the resonances, decrease
or increase the beat note, since the split combs will see different cavity round-trip times. Such reso-
nant structures that applies to multiple teeth of the comb can be passive resonators (Fabry-Perot) in
transmission or reflection, narrow atomic two-photon resonances, phase matched frequency dou-
blers, etc. . . . Experimental demonstrations will be presented. No correlation is observed between
pulse velocity and beat note enhancement/reduction.
Mode-locked fiber lasers are the most promising lasers for intracavity phase interferometry,1 because they offer the possibility to have two orthogonally polarized pulses circulating independently in the cavity. The saturable absorbers based on polarization maintaining tapered fiber coated with carbon nanotubes are developed and analyzed for minimum coupling between the slow and fast axis of the fiber.
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