Michael Steinke

Gain dynamics in Er 3+ :Yb 3+ co-doped fiber amplifiers

Gain dynamics of Er:Yb fiber amplifiers were studied analytically and corresponding transfer functions were measured, showing good agreement with the theoretical treatment. In addition, numerical investigations have been carried out to get deeper insight.

Single-frequency fiber amplifier at 1.5 µm with 100 W in the linearly-polarized TEM 00 mode for next-generation gravitational wave detectors

Next-generation gravitational wave detectors require single-frequency and high power lasers at a wavelength of 1.5 µm addressing a set of demanding requirements such as linearly-polarized TEM00 radiation with low noise to run for long periods. In this context, fiber amplifiers in MOPA configuration are promising candidates to fulfill these requirements. We present a single-frequency monolithic Er:Yb co-doped fiber amplifier (EYDFA) at 1.5 µm with a linearly-polarized TEM00 output power of 100 W. The EYDFA is pumped off-resonant at 940 nm to enhance the Yb-to-Er energy transfer efficiency and enable higher ASE threshold. We also performed numerical simulations to investigate the off-resonant pumping scheme and confirm the corresponding experimental results.

Influence of the third energy level on the gain dynamics of EDFAs: analytical model and experimental validation

We report an analytical model and experimental validation of the temporal dynamics of 3-level system fiber amplifiers. The model predictions show a good agreement with the measured pump power to output power and the pump power to output phase transfer functions in an EDFA pumped at 976 nm, as well as with the typical literature values for the spontaneous lifetime of the involved energy levels. The measurements show a linear relation between the effective lifetime of the meta-stable level and the output power, and a filtering of the temperature-induced phase-shift due to the quantum defect at a sufficiently high frequency modulation.

Gain dynamics in Raman fiber lasers and passive pump-to-Stokes RIN suppression.

We report on theoretical and experimental investigations of gain dynamics in Raman fiber lasers in the frequency range of 1 Hz-1 MHz. An analytical solution of the problem is due to the nonlinear nature of the Raman effect not feasible. Thus, we used a numerical simulation to gain general insights. Experimentally and numerically obtained results for a Raman fiber laser emitting at 1180 nm show good qualitative agreement. We also present a potential physical interpretation of the observed dynamical properties. In addition, we report on an experimental proof-of-principle of a passive pump-to-Stokes RIN suppression scheme for the main Stokes order in cascaded Raman fiber lasers utilizing an additional parasitic Stokes order. Again, results from numerical and experimental studies of a cascaded Raman fiber laser at 1180 nm and 1240 nm show good agreement and confirm the passive pump-to-Stokes RIN suppression at 1180 nm. The dependencies between the resonator design and the parameters of the noise suppression are investigated. In addition, it is shown that the scheme can also be applied to cascaded Raman fiber lasers with more then two Stokes shifts. This opens the possibility to design for example low-noise Raman fiber lasers at 1480 nm to pump low-noise Er(3+) doped fiber amplifiers.

Long-Period Gratings in Highly Germanium-Doped, Single-Mode Optical Fibers for Sensing Applications

Long-period fiber gratings (LPGs) are well known for their sensitivity to external influences, which make them interesting for a large number of sensing applications. For these applications, fibers with a high numerical aperture (i.e., fibers with highly germanium (Ge)-doped fused silica fiber cores) are more attractive since they are intrinsically photosensitive, as well as less sensitive to bend- and microbend-induced light attenuations. In this work, we introduce a novel method to inscribe LPGs into highly Ge-doped, single-mode fibers. By tapering the optical fiber, and thus, tailoring the effective indices of the core and cladding modes, for the first time, an LPG was inscribed into such fibers using the amplitude mask technique and a KrF excimer laser. Based on this novel method, sensitive LPG-based fiber optic sensors only a few millimeters in length can be incorporated in bend-insensitive fibers for use in various monitoring applications. Moreover, by applying the described inscription method, the LPG spectrum can be influenced and tailored according to the specific demands of a particular application.

Manufacturing and characterization of asymmetric evanescent field polished fiber couplers for fiber grating assisted mode selective coupling

Mode division multiplexing (MDM) could bring a technological progress in the field of optical telecommunication by increasing the data transmission bandwidth. A key challenge for enabling MDM lies in manufacturing of efficient and cost-effective mode–selective fiber couplers. The fiber grating based mode selective coupling approach is a method that is currently being under research in this context. In this work a novel process for manufacturing of asymmetric evanescent field polished couplers is presented which enables grating assisted mode selective coupling. In addition, we discuss the optical setup developed for characterization of these couplers.

Development of a comprehensive 3D model for transversal mode instability investigations

We have developed a comprehensive 3D steady-state numerical model of high power Yb-doped fiber amplifiers which allows the investigation of thermally induced multimode behavior being the reason for the transversal mode instabilities phenomenon. Numerical simulations show that for the pump powers above 3 kW the refractive index grating inscribed as a result of thermo-optic effect supports an efficient energy transfer from the main mode to high order modes. Coupling of the initial Gaussian beam without any shift or tilt to the investigated amplifier leads to the preferential excitation and further growth of symmetrical high order modes (LP02, LP03). Anti-symmetrical modes are not growing. Increasing the pump power leads to stronger mode coupling which is related to the increase of the amplitude and frequency of the temperature oscillations and corresponding refractive index oscillations over the longitudinal axis of the fiber amplifier.

Monolithic fiber amplifiers for the next generation of gravitational wave detectors

Single-frequency Yb3+ and Er3+:Yb3+ fiber amplifiers (YDFA/EYDFA) in MOPA configuration operating at 1064 nm and around 1550 nm are promising candidates to fulfill the challenging requirements on laser sources for the next generation of interferometric gravitational wave detectors (GWDs). They offer high beam quality, long-term stability and allow for excellent thermal management. We developed an engineering fiber amplifier prototype at 1064 nm emitting around 200W of linearly-polarized light in the TEM00 mode. The system consists of three modules: the seed source, the pre-amplifier and the main amplifier. The modular design ensures reliable long-term operation, decreases system complexity and simplifies maintenance procedures and repair. In addition, commercial available fibers increase the flexibility of the entire system. We also developed and characterized a fiber amplifier prototype at 1556 nm that emits 100W of linearly-polarized light in the TEM00 mode. The EYDFA is pumped off-resonantly at 940 nm to enhance the Yb3+-to-Er3+ energy transfer efficiency and enable a higher amplified spontaneous emission (ASE) threshold. In addition to that, we performed measurements to study phase to intensity noise coupling via the Kramers-Kronig relation above the stimulated Brillouin scattering (SBS) threshold, as it was proposed based on numerical simulations. This effect is based on an asymmetric gain spectrum, which we measured experimentally and used for the reconstruction of the broadband excess intensity noise.

Versatile monolithic 2-micron laser systems

To answer a growing demand in development of high power pulsed and continuous wave sources at 2 micron spectral range we have participated in several projects, which resulted in a delivery of versatile monolithic sources providing picosecond, nanosecond and CW laser signal. As an example of pulsed sources we developed all-fiber monolithic devices based on a directly modulated laser diode and gain-switched laser diode to generate nanosecond and picosecond pulses, respectively, which are amplified in the same fiber amplifier chain up to 50 µJ with 96 ps and more than 1 mJ with pulses longer than 35 ns.

Recent progress on monolithic fiber amplifiers for next generation of gravitational wave detectors

Single-frequency fiber amplifiers in MOPA configuration operating at 1064 nm (Yb3+) and around 1550 nm (Er3+ or Er3+:Yb3+) are promising candidates to fulfill the challenging requirements of laser sources of the next generation of interferometric gravitational wave detectors (GWDs). Most probably, the next generation of GWDs is going to operate not only at 1064 nm but also at 1550 nm to cover a broader range of frequencies in which gravitational waves are detectable. We developed an engineering fiber amplifier prototype at 1064 nm emitting 215 W of linearly-polarized light in the TEM00 mode. The system consists of three modules: the seed source, the pre-amplifier, and the main amplifier. The modular design ensures reliable long-term operation, decreases system complexity and simplifies repairing and maintenance procedures. It also allows for the future integration of upgraded fiber amplifier systems without excessive downtimes. We also developed and characterized a fiber amplifier prototype at around 1550 nm that emits 100 W of linearly-polarized light in the TEM00 mode. This prototype uses an Er3+:Yb3+ codoped fiber that is pumped off-resonant at 940 nm. The off-resonant pumping scheme improves the Yb3+-to-Er3+ energy transfer and prevents excessive generation of Yb3+-ASE.