Quantum-limited measurements of intensity noise levels in Yb-doped fiber amplifiers
Alexandra Popp, Victor Distler, Kevin Jaksch, Florian Sedlmeir, Christian R. Müller, Nicoletta Haarlammert, Thomas Schreiber, Christoph Marquardt, Andreas Tünnermann, Gerd Leuchs
We investigate the frequency-resolved intensity noise spectrum of an Yb-doped fiber amplifier down to the fundamental limit of quantum noise. We focus on the kHz and low MHz frequency regime with special interest in the region between 1 and 10 kHz. Intensity noise levels up to >60 dB above the shot noise limit are found, revealing great optimization potential. Additionally, two seed lasers with different noise characteristics were amplified, showing that the seed source has a significant impact and should be considered in the design of high power fiber amplifiers.
We describe a method to measure the spectral radiance of a source in an absolute way without the need of a reference. Here we give the necessary detail to allow for the device to be reproduced from standard fiber-optic components. The device is suited for fiber-optic applications at telecom wavelengths and calibration of powermeters and spectrometers at light levels from 1nW to 1uW.
The dynamics of the state of polarization in single mode and multimode fiber amplifiers are presented. The experimental results reveal that although the state of polarizations at the output can vary over a large range when changing the temperatures of the fiber amplifiers, the variations are significantly reduced when resorting to the principal states of polarization in single mode fiber amplifiers and principal modes in multimode fiber amplifiers.
We use a detailed numerical model of stimulated thermal Rayleigh scattering to compute mode instability thresholds in Tm$^{3+}$-doped fiber amplifiers. The fiber amplifies 2040 nm light using a 790 nm pump. The cross-relaxation process is strong, permitting power efficiencies of 60%. The predicted instability thresholds are compared with those in similar Yb$^{3+}$-doped fiber amplifiers with 976 nm pump and 1060 nm signal, and are found to be higher, even though the heat load is much higher in Tm-doped amplifiers. The higher threshold in the Tm-doped fiber is attributed to its longer signal wa…
We demonstrate a 7-core erbium doped fiber amplifier employing side pumping using tapered multimode fiber. The amplifier has multicore inputs and outputs which can be readily spliced to multicore transmission fiber for amplifying space division multiplexed signals. Gain over 25dB was obtained in each of the cores over a 40-nm bandwidth covering C-band.
We show that nonlinear phase shifts and third-order dispersion can compensate each other in short-pulse fiber amplifiers. In particular, we consider chirped-pulse fiber amplifiers at wavelengths for which the fiber dispersion is normal. The nonlinear phase shift accumulated in the amplifier can be compensated by the third-order dispersion of the combination of a fiber stretcher and grating compressor. A numerical model is used to predict the compensation, and initial experimental results that exhibit the main features of the calculations are presented. In the presence of third-order dispersion…
We experimentally analyze the average-power-scaling capabilities of ultrafast, thulium-doped fiber amplifiers. It has been theoretically predicted that thulium-doped fiber laser systems, with an emission wavelength around 2 um, should be able to withstand much higher heat-loads than their Yb-doped counterparts before the onset of transverse mode instability (TMI) is observed. In this work we experimentally verify this theoretical prediction by operating thulium doped fibers at very high heat-load. In separate experiments we analyze the performance of two different large-core, thulium-doped fib…
A theoretical model for laser cooled continuous-wave fiber amplifier is presented. The amplification process takes place in the Tm3+-doped core of the fluoride ZBLAN (ZrF4-BaF2-LaF3-AlF3-NaF) glass fiber. The cooling process takes place in the Yb3+:ZBLAN fiber cladding. It is shown that for each value of the pump power and the amplified signal there is a distribution of the concentration of the Tm3+ along the length of the fiber amplifier, which provides its athermal operation. The influence of a small deviation in the value of the amplified signal on the temperature of the fiber with the fixe…
We numerically model fiber amplifier performance near and slightly above the mode instability threshold. These results are compared with recently published experimental work. Using weakly amplitude modulated pump light we obtain qualitative agreement with the measured instability thresholds, mode switching ranges, pixel power modulations, and modal amplitude modulations.
Using a numerical model we study the frequency dependence of mode coupling gain due to stimulated thermal Rayleigh scattering in step index, Yb doped, fiber amplifiers. The frequency at the gain peak is shown to vary with core size, doping size, population saturation, thermal lensing, fiber coiling, direction of pumping, photodarkening, and pump noise spectra. The predicted frequencies are compared with measured values whenever possible.
A stable, continuous wave, single frequency fiber amplifier system at 1015 nm with 10W output power is presented. It is based on a large mode double clad fiber cooled to liquid nitrogen temperature. The amplified light is frequency quadrupled to 254 nm and used for spectroscopy of the 6^1S - 6^3P transition in mercury.
Using the Bogoliubov transformation in multi-frequency modes to describe the evolution of the non-degenerate signal and idler twin beams, we theoretically investigate the quantum noise properties of the pulse pumped high gain fiber optical parametric amplifiers (FOPA). The results show that the noise figure of the FOPA is generally greater than the 3 dB quantum limit unless the joint spectral function is factorable and the spectrum of the input signal well matches the gain spectrum in the signal band. However, the intensity difference noise of the twin beams, which weakly depends on the joint …
A Core/Cladding Yb-doped fiber amplifier configuration is presented that relies on anti-Stokes fluorescence cooling for effective heat mitigation in high-power operation. In the proposed design, the inner cladding of the double clad fiber is doped with the same ion as in the core; therefore, the excess heat generated from the background absorption is removed by the anti-Stokes fluorescence in both core and inner cladding. We consider both silica and ZBLAN glasses for the host material in the Core/Cladding Yb-doped fiber amplifier. The model incorporates the spatial profiles of the signal and p…
We show by numerical modeling that saturation of the population inversion reduces the stimulated thermal Rayleigh gain relative to the laser gain in large mode area fiber amplifiers. We show how to exploit this effect to raise mode instability thresholds by a substantial factor. We also demonstrate that when suppression of stimulated Brillouin scattering and the population saturation effect are both taken into account, counter-pumped amplifiers have higher mode instability thresholds than co-pumped amplifiers for fully Yb$^{3+}$ doped cores, and confined doping can further raise the thresholds…
We show how signal linewidth affects the gain of stimulated thermal Rayleigh scattering (STRS) which is responsible for mode instability in fiber amplifiers. The gain is reduced if the coherence time of the signal is less than the group velocity induced walk off between modes LP$_{01}$ and LP$_{11}$. We derive equations for short pulses, linear chirps, and general periodic cases.
One of the important function in optical communication system is the distribution of information encoded in an optical beam. It is not a problem to accomplish this in a classical system since classical information can be copied at will. However, challenges arise in quantum system because extra quantum noise is often added when the information content of a quantum state is distributed to various users. Here, we experimentally demonstrate a quantum information tap by using a fiber optical parametric amplifier (FOPA) with correlated inputs, whose noise is reduced by the destructive quantum interf…
We report on detailed in-situ distributed temperature measurements inside a high power fiber amplifier. The deducted thermal load and the TMI-threshold of a commercial LMA fiber with 25 micron core and 400 micron cladding was measured at various seed wavelengths. By matching these results with detailed simulations we show that photodarkening has a negligible impact on the thermal load and, therefore, on the TMI threshold in this fiber.
We show by detailed numerical modeling that stimulated thermal Rayleigh scattering can account for the modal instability observed in high power fiber amplifiers. Our model illustrates how the instability threshold power can be maximized by eliminating amplitude and phase modulation of the signal seed and the pump, and by careful launch of the signal seed. We also illustrate the influence of photodarkening and mode specific loss on the threshold.
We present a new scheme for a laser cooled Raman optical fiber amplifier. The heat generated in the process of stimulated Raman scattering is compensated for with laser cooling provided by anti-Stokes fluorescence of ytterbium ions doped in the core of the fiber. The device is pumped with two pump sources. One of the pump sources provides Raman amplification of the signal and does not interact with the ytterbium ions in the core of the fiber. The second pump provides laser cooling of the fiber with anti-Stokes fluorescence of the Yb3+ ions. The proper arrangement of Yb3+distribution can provid…
Kilowatt-level high efficiency all-fiberized Raman fiber amplifier based on pure passive fiber is proposed for the first time in this paper. The laser system is established on master oscillator power amplification configuration while a piece of graded-index passive fiber is utilized as stokes shifting as well as gain medium, which is entirely irrelevant to rare-earth-doped gain mechanism. When the pump power is 1368.8 W, we obtained 1002.3 W continuous-wave laser power at 1060 nm with the corresponding optical-to-optical efficiency of 84%. The beam parameter M2 improves from 9.17 of the pump l…
In this letter, the impact of pump configuration on the modal instabilities (MI) threshold of high power fiber amplifiers is investigated. Using a time dependent computer model based on the optical beam propagation method (BPM), the effect of bidirectional pumping on the MI threshold of high power fiber amplifiers is explored and compared to that of a co-pumped configuration. Simulation results indicate that the bidirectional pumping increases the instabilities threshold by a factor of 30% with respect to the co-pumped scheme. We attribute this threshold enhancement to a weaker strength of the…
Recent experimental results have reported the observation of beam self-cleaning or, more generally, nonlinear beam reshaping in active multimode fibers. In this work we present a numerical analysis of these processes, by considering the ideal case of a diode-pumped signal amplifier made of a graded-index multimode fiber with uniform Yb doping. Simulations confirm that beam cleaning of the signal may take place even in amplifying fibers, that is the absence of beam energy conservation. Moreover, we show how the local signal intensity maxima, which are periodically generated by the self-imaging …
We present a high power single-frequency ytterbium fiber amplifier system with an output power of 30 W at 1091 nm. The amplifier system consists of two stages, a preamplifier stage in which amplified spontaneous emission is efficiently suppressed (>40 dB) and a high power amplifier with an efficiency of 52 %. Two different approaches of frequency doubling are compared. We achieve 8.6 W at 545.5 nm by single-pass frequency doubling in a MgO-doped periodically poled stoichiometric LiTaO3 and up to 19.3 W at 545.5 nm by frequency doubling with a lithium-triborate (LBO) crystal in an external enha…
We present a passively mode-locked, tunable soliton Ho:fiber ring oscillator, optimized for seeding of Ho:YLF amplifiers. The oscillator is independently tunable in central wavelength and spectral width from 2040 nm to 2070 nm and from 5 nm to 10 nm, respectively. At all settings the pulse energy within the soliton is around 800 pJ. The soliton oscillator was optimized to fully meets the spectral requirements for seeding Ho:YLF amplifiers. Its Kelly sidebands are located outside the amplifier gain spectrum, resulting in a train of about 1 ps long pedestal-free pulses with relative intensity no…
By focusing on a typical emitting wavelength of 1120 nm as an example, we present the first demonstration of a high-efficiency, narrow-linewidth kilowatt-level all-fiber amplifier based on hybrid ytterbium-Raman (Yb-Raman) gains. Notably, two temporally stable, phase-modulated single-frequency lasers operating at 1064 nm and 1120nm, respectively, were applied in the fiber amplifier, to alleviate the spectral broadening of the 1120 signal laser and suppress the stimulated Brillouin scattering (SBS) effect simultaneously. Over 1 kW narrow-linewidth 1120 nm signal laser was obtained with a slope …
Lithium niobate on insulator (LNOI), as an emerging and promising optical integration platform, faces shortages of on-chip active devices including lasers and amplifiers. Here, we report the fabrication on-chip erbium-doped LNOI waveguide amplifiers based on electron beam lithography and inductively coupled plasma reactive ion etching. A net internal gain of ~30 dB/cm in communication band was achieved in the fabricated waveguide amplifiers under the pump of a 974-nm continuous laser. This work develops new active devices on LNOI and will promote the development of LNOI integrated photonics.
We report on the influence of core NA on thermal-induced mode instabilities (MI) in high power fiber amplifiers. Influence of core NA and V-parameter on MI has been investigated numerically. It shows that core NA has larger influence on MI for fibers with smaller core-cladding-ratio, and the influence of core NA on threshold is more obvious when the amplifiers are pumped at 915nm. The dependence of threshold on V-parameter revealed that the threshold increases linearly as V-parameter decreases when V-parameter is larger than 3.5, and the threshold shows exponentially increase as V-parameter de…
We study continuous-wave light propagation through a twisted birefringent single-mode fiber amplifier with saturable nonlinearity. The corresponding coupled-mode system is isomorphic to a non-Hermitian nonlinear dimer, and gives rise to analytic polarization-mode dynamics. It provides an optical simulation of the semi-classical non-Hermitian Bose-Hubbard model, and suggests its use for the design of polarization circulators and filters, as well as sources of polarized light.
Dependence of mode instabilities (MI) on ytterbium dopant concentrations in high power fiber amplifiers has been investigated. It is theoretically shown that, by only varying the fiber length to maintain the same total small-signal pump absorption, the MI threshold is independent of dopant concentration. MI thresholds of gain fibers with ytterbium dopant concentration of 5.93X10^25/m3 and 1.02X10^26/m3 have been measured, which exhibit similar thresholds and agree with theoretical results. The result indicates that heavy doping of active fiber can be adopted to suppress nonlinear effects witho…
Lithium niobate on insulator (LNOI) is an emerging photonic platform with great promises for future optical communications, nonlinear optics and microwave photonics. An important integrated photonic building block, active waveguide amplifiers, however, is still missing in the LNOI platform. Here we report an efficient and compact waveguide amplifier based on erbium-doped LNOI waveguides, realized by a sequence of erbium-doped crystal growth, ion slicing and lithography-based waveguide fabrication. Using a compact 5-mm-long waveguide, we demonstrate on-chip net gain of > 5 dB for 1530-nm signal…
A novel method for efficient generation of high power, equalized continuous-wave supercontinuum source in an all conventional silica fiber architecture is demonstrated. Highly nonlinear fiber (HNLF) is pumped in its anomalous dispersion region using a novel, high power, L-band laser. The L-band laser encompasses a 6th order cascaded Raman amplifier which is pumped with a high power Ytterbium doped fiber laser and amplifies a low-power, tunable L-band seed source. The supercontinuum generated 35W of power with ~40% efficiency. The Supercontinuum spectrum was measured to have a high degree of fl…
We describe a fiber Raman amplifier for nanosecond and sub-nanosecond pulses centered around 1260 nm. The amplification takes place inside a 4.5-m-long polarization-maintaining phosphorus-doped fiber, pumped at 1080 nm by 3-ns-long pulses with a repetition rate of 200 kHz and up to 1.75 kW peak power. The input seed pulses are of sub-mW peak-power and minimal duration of 0.25 ns, carved off a continuous-wave laser with sub-MHz linewidth. We obtain linearly-polarized output pulses with peak-powers of up to 1.4 kW, corresponding to peak-power conversion efficiency of over 80%. An ultrahigh small…
Integrated optical amplifiers and light sources are of great significance for photonic integrated circuits (PICs) and have attracted many research interests. Doping rare-earth ions in materials as a solution to realize efficient optical amplifiers and lasing has been investigated a lot. We investigate the erbium-doped lithium niobate on insulator (LNOI). Here, spiral waveguide amplifiers were fabricated on a 1-mol\% erbium-doped LNOI by CMOS-compatible technique. We demonstrated a maximum internal net gain of 8.3 dB at 1530 nm indicating a net gain per unit length of 15.6 dB/cm with a compact …
We develop a high-power narrow-linewidth fiber amplifier based on a tapered Yb-doped fiber. In the experiment, the narrow end of the tapered fiber is used as the input port of both pump source and signal light to form a robust all-fiber configuration. The stimulated Brillouin scattering (SBS) is effectively suppressed owing to its gradually increased mode area as well as continuously changed Brillouin frequency shift dependent on the core diameter. As a result, an output power of 260 W without SBS and mode instability (MI) is obtained, with a slope efficiency of 71.3 % and a narrow linewidth o…
We demonstrate wavelength-wavelength correlations of classical broad-band amplified spontaneous emission (ASE) photons emitted by an erbium-doped fiber amplifier (EDFA) in a wavelength regime around 1530nm. We then apply these classical correlated photons in the framework of a real-world ghost spectroscopy experiment at a wavelength of 1533nm to acetylene (C2H2) reproducing the characteristic absorption features of the C-H stretch and rotational bands. This proof-of-principle experiment confirms the generalization of an ASE source concept offering an attractive light source for classical ghost…
We demonstrated all-fiber amplification of 11 ps pulses from a gain-switched laser diode at 1064 nm. The diode was driven at a repetition rate of 40 MHz and delivered 13 $μ$W of fiber-coupled average output power. For the low output pulse energy of 325 fJ we have designed a multi-stage core pumped pre-amplifier in order to keep the contribution of undesired amplified spontaneous emission as low as possible. By using a novel time-domain approach for determining the power spectral density ratio (PSD) of signal to noise, we identified the optimal working point for our pre-amplifier. After the pre…
We estimate the Stokes wave starting power for stimulated thermal Rayleigh scattering (STRS) produced by thermal fluctuations in the fiber core that transiently alter the refractive index profile in the core. A transverse temperature gradient creates a transverse refractive index gradient via the thermo optic effect, and if the fluctuation frequency lies in the STRS gain band, it can couple light from mode LP$_{01}$ to LP$_{11}$ to seed STRS. This spontaneous Rayleigh seed may be stronger than the quantum background and may affect the mode instability thresholds of fiber amplifiers. This new s…
Photonic crystal fibers are used in fiber amplifiers and lasers because of the flexibility in the design of mode area and dispersion. However, these quantities depend strongly on the wavelength. The wavelength dependence of gain, nonlinearity and dispersion are investigated here by including the wavelength dependence explicitly in the nonlinear Schrödinger equation for photonic crystal fibers with varying periods and hole sizes. The effect of the wavelength dependence of each parameter is studied separately as well as combined. The wavelength dependence of the parameters is shown to create asy…
The influence of low temperatures on the performance of a high-power single-frequency fiber laser amplifier is evaluated with a numerical simulation. Cooling the fiber can allow to take advantage of both higher damping of the acoustic waves in the silica glass, and higher laser efficiency. We first report on the measurement of the stimulated Brillouin scattering (SBS) threshold in a silica fiber as a function of the temperature from 300~K down to 77~K. We then present the measurements of small-signal absorption and gain spectra of an erbium doped alumino-phosphosilicate fiber, at room temperat…
In this work, the spectral evolution properties in single-frequency Raman fiber amplifier (RFA) with different pump manners are analyzed theoretically for the first time based on the gain dynamics. The analysis of gain dynamics reveals that the walk-off effect in counter-pumped manner produces a natural low-pass filter in single-frequency RFA. When applying rare-earth doped fiber lasers as the pump source, the strong temporal fluctuations in the pump source lead to spectral broadening in co-pumped manner, while the natural low-pass filter in countered-pumped case can still ensure single-freque…
Erbium-doped lithium niobate on insulator (Er:LNOI) has attracted enormous interest as it provides gain and enables integrated amplifiers and lasers on the lithium niobate on insulator (LNOI) platform. We demonstrate a highly efficient waveguide amplifier on Er:LNOI. The 2.58-cm long amplifier can achieve 27.94 dB signal enhancement, 16.0 dB internal net gain (6.20 dB/cm), -8.84 dBm saturation power, 4.59 dB/mW power conversion efficiency, and 4.49 dB noise figure at 1531.6 nm. Besides, thorough investigation on the pumping wavelength, pumping scheme, output power and noise figure have been pe…
In long-haul optical continuous-wave frequency transfer via fiber, remote bidirectional Er$^+$-doped fiber amplifiers are commonly used to mitigate signal attenuation. We demonstrate for the first time the ultrastable transfer of an optical frequency using a remote fiber Brillouin amplifier, placed in a server room along the link. Using it as the only means of remote amplification, on a 660 km loop of installed underground fiber we bridge distances of 250 km and 160 km between amplifications. Over several days of uninterrupted measurement we find an instability of the frequency transfer (Allan…
We present a detailed description of the methods used in our model of mode instability in high-power, rare earth-doped, large-mode-area fiber amplifiers. Our model assumes steady-periodic behavior, so it is appropriate to operation after turn on transients have dissipated. It can be adapted to transient cases as well. We describe our algorithm, which includes propagation of the signal field by fast-Fourier transforms, steady-state solutions of the laser gain equations, and two methods of solving the time-dependent heat equation: alternating-direction-implicit integration, and the Green's funct…
We study self-similar dynamics of picosecond light pulses generating in optical fiber amplifiers and fiber lasers with distributed parameters. A rich variety of periodic and solitary wave solutions are derived for the governing generalized nonlinear Schrödinger equation with varying coefficients in the presence of gain effect. The constraint on distributed optical fiber parameters for the existence of these wave solutions is presented. The dynamical behaviour of those self-similar waves is discussed in a periodic distributed amplification system. The stability of periodic and solitary wave sol…
We report what we believe to be the first radiation-balanced fiber amplifier - a device that provides optical gain while experiencing no temperature rise. The gain medium is a silica fiber with a 21 um-diameter core highly doped with Yb3+ (2.52 wt.%) and co doped with 2.00 wt.% Al to reduce concentration quenching. The amplifier was core-pumped with 1040 nm light to create anti-Stokes fluorescence (ASF) cooling and gain in the core at 1064 nm. Using a custom slow-light FBG sensor with mK resolution, temperature measurements were performed at multiple locations along the amplifier fiber. A 4.35…
The interplay of such cornerstones of modern nonlinear fiber optics as a nonlinearity, stochasticity and polarization leads to variety of the noise induced instabilities including polarization attraction and escape phenomena harnessing of which is a key to unlocking the fiber optic systems specifications required in high resolution spectroscopy, metrology, biomedicine and telecommunications. Here, by using direct stochastic modeling, the mapping of interplay of the Raman scattering-based nonlinearity, the random birefringence of a fiber, and the pump-to-signal intensity noise transfer has been…
We experimentally investigate the evolution of the direct detection noise figure of a non degenerate phase sensitive amplifier based on nonlinear fiber, as a function of the relative phase between the signal, idler, and pump, all other parameters remaining fixed. The use of a fiber with a high stimulated Brillouin scattering threshold permits to investigate the full range of phase sensitive gain and noise figure without pump dithering. Good agreement is found with theory, both for signal only and combined signal and idler direct detections.
