Optical cooling of a Yb-doped alumino-phosphosilicate fiber in air by -250†mK.

Recent progress in the fabrication of Yb-doped silicate fibers with low concentration quenching and low background absorption loss has led to the demonstration of anti-Stokes-fluorescence cooling in several aluminosilicate compositions. This breakthrough is critical to combat deleterious thermal effects due to the quantum defect in fiber lasers and amplifiers. Since cooling efficiencies remain low (1-2.7%), it is paramount to engineer compositions that improve this metric. We report a silica fiber with a core glass heavily doped with aluminum and phosphorus that sets, to our knowledge, a few new records. This few-mode fiber (16-µm core) was cooled in air by -0.25 K from room temperature with ∼0.5 W of 1040-nm power. The measured cooling efficiency is 3.3% at low pump power and 2.8% at the power that produced maximum cooling. The critical quenching concentration inferred from the measured dependence of cooling on pump power and careful calibration of the pump absorption and saturation is 79 wt.%. The inferred background absorption loss is 15 dB/km. Together with the fiber's average Yb concentration of 4.2 wt.%, these metrics rank among the best reported in a silica glass.

High-efficiency radiation-balanced Yb-doped silica fiber laser with 200-mW output.

The focus of this study was the development of a second generation of fiber lasers internally cooled by anti-Stokes fluorescence. The laser consisted of a length of a single-mode fiber spliced to fiber Bragg gratings to form the optical resonator. The fiber was single-moded at the pump (1040 nm) and signal (1064 nm) wavelengths. Its core was heavily doped with Yb, in the initial form of CaF2 nanoparticles, and co-doped with Al to reduce quenching and improve the cooling efficiency. After optimizing the fiber length (4.1 m) and output-coupler reflectivity (3.3%), the fiber laser exhibited a threshold of 160 mW, an optical efficiency of 56.8%, and a radiation-balanced output power (no net heat generation) of 192 mW. On all three metrics, this performance is significantly better than the only previously reported radiation-balanced fiber laser, which is even more meaningful given that the small size of the single-mode fiber core (7.8-µm diameter). At the maximum output power (∼2 W), the average fiber temperature was still barely above room temperature (428 mK). This work demonstrates that with anti-Stokes pumping, it is possible to induce significant gain and energy storage in a small-core Yb-doped fiber while keeping the fiber cool.

Anti-Stokes fluorescence cooling of nanoparticle-doped silica fibers.

The first observation of cooling by anti-Stokes pumping in nanoparticle-doped silica fibers is reported. Four Yb-doped fibers fabricated using conventional modified chemical vapor deposition (MCVD) techniques were evaluated, namely, an aluminosilicate fiber and three fibers in which the Yb ions were encapsulated in CaF2, SrF2, or BaF2 nanoparticles. The nanoparticles, which oxidize during preform processing, provide a modified chemical environment for the Yb3+ ions that is beneficial to cooling. When pumped at the near-optimum cooling wavelength of 1040 nm at atmospheric pressure, the fibers experienced a maximum measured temperature drop of 20.5 mK (aluminosilicate fiber), 26.2 mK (CaF2 fiber), and 16.7 mK (SrF2 fiber). The BaF2 fiber did not cool but warmed slightly. The three fibers that cooled had a cooling efficiency comparable to that of the best previously reported Yb-doped silica fiber that cooled. Data analysis shows that this efficiency is explained by the fibers' high critical quenching concentration and low residual absorptive loss (linked to sub-ppm OH contamination). This study demonstrates the large untapped potential of nanoparticle doping in the current search for silicate compositions that produce optimum anti-Stokes cooling.

Rotation sensitivity and shot-noise-limited detection in an exceptional-point coupled-ring gyroscope.

A theoretical study is performed of the sensitivity and quantum-noise limit of a passive coupled-ring optical gyroscope operated at and detuned from its exceptional point (EP) and interrogated with a practical conventional readout system. When tuned to its EP, the Sagnac frequency splitting is proportional to the square root of the applied rotation rate, but the signal generated by the sensor is shown to be proportional to the applied rotation rate. The sensitivity is never larger, and the minimum detectable rotation rate in the quantum-noise limit never lower, than that of a standard single-ring gyro of the same radius and loss, even when the coupled-ring gyro is tuned exactly to its EP. As pointed out elsewhere for other EP sensors, in this particular passive sensor at least, there is no sensitivity or resolution benefit in operating at an EP.

Radiation-Balanced Silica Fiber Amplifier.

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-µm-diameter core highly doped with Yb^{3+} (2.52 wt. %) and codoped with 2.00 wt. % Al to reduce concentration quenching. The amplifier is core pumped with 1040-nm light to create anti-Stokes fluorescence cooling and gain in the core at 1064 nm. Using a custom slow-light fiber Bragg grating sensor with mK resolution, temperature measurements are performed at multiple locations along the amplifier fiber. A 4.35-m fiber pumped with 2.62 W produced 17 dB of gain, while the average fiber temperature remained slightly below room temperature. This advancement is a fundamental step toward the creation of ultrastable lasers necessary to many applications, especially low-noise sensing and high-precision metrology.

Enhanced rotation sensing and exceptional points in a parity-time-symmetric coupled-ring gyroscope.

Enhancement in rotation sensitivity is achieved in a parity-time-symmetric gyroscope consisting of a ring with gain coupled to a lossy ring, operated below laser threshold and in the vicinity of its exceptional point (EP). An external laser and a conventional readout system are used to measure the large rotation-induced shifts in resonance frequency known to occur in this device. A complete model of the rotation sensitivity is derived that accounts for gain saturation caused by the large circulating power. Compared to a single-ring gyro, the sensitivity is enhanced by a factor of ∼300 when the inter-ring coupling is tuned to its EP value κEP, and ∼2400 when it is decreased from κEP, even though the Sagnac frequency shift is then much smaller. ∼40% of this 2400-fold enhancement is assigned to a new sensing mechanism where rotation alters the gain saturation. These results show that this compact gyro has a far greater sensitivity than a conventional ring gyro, and that this improvement arises mostly from the gain compensating the loss, as opposed to the enhanced Sagnac frequency shift from the EP. This gyro is also shown to be much more stable against gain fluctuations than a single-ring gyro with gain.

Compact diaphragm-based optical accelerometers with µg/√Hz resolution.

A compact fiber accelerometer that meets the resolution requirement for aircraft navigation is reported. It detects extremely weak acceleration-induced vibrations of a spring-loaded diaphragm using a two-wave interferometer with a π/2 biasing step micro-fabricated on the diaphragm. A single-mode fiber provides a laser beam that interrogates the interferometer. This sensor has a measured flat-band sensitivity with a bandwidth of 10.7 kHz, and a resolution limited by thermo-mechanical noise of 13µg/√Hz at 100 Torr, and 712ng/√Hz at 20 mTorr.

Experimental comparison of silica fibers for laser cooling.

Laser cooling in silica has recently been demonstrated, but there is still a lack of understanding on how fiber composition, core size, and OH- contamination influence cooling performance. In this work, six Yb-doped silica fibers were studied to illuminate the influence of these parameters. The best fiber cooled by -70mK with only 170 mW/m of absorbed pump power at 1040 nm, which corresponds to twice as much heat extracted per unit length compared to the first reported laser cooling in silica. This new fiber has an extremely low OH- loss and a higher Al concentration (2.0 wt.% Al), permitting a high Yb concentration (2.52 wt.% Yb) without incurring significant quenching. Strong correlations were found between the absorptive loss responsible for heating and the loss measured at 1380 nm due to absorption by OH-.

Loss-compensated slow-light fiber Bragg grating with 22-km/s group velocity.

This Letter reports the behavior of the slow-light resonances of a strong apodized fiber Bragg grating (FBG) in which the intrinsic loss is compensated for by a small internal gain. The 6.5-mm FBG, written with a femtosecond laser in an Er-doped single-mode fiber, was pumped at ∼1475nm just below the lasing threshold to offset most of its intrinsic loss, thereby narrowing its resonances. The fundamental slow-light resonance was measured to have a linewidth of 8.5 fm, or a record group velocity of ∼22km/s, and a peak transmission near unity (-0.2dB). The measured dependencies of the linewidth and peak transmission on pump power agree well with a new model that predicts the transmission spectrum of loss-compensated FBGs in the presence of pump and signal saturation.

Laser cooling in a silica optical fiber at atmospheric pressure.

For the first time, to the best of our knowledge, laser cooling is reported in a silica optical fiber. The fiber has a 21-µm diameter core doped with 2.06 wt.% ${{\rm Yb}^{3 + }}$Yb3+ and co-doped with ${{\rm Al}_2}{{\rm O}_3}$Al2O3 and ${{\rm F}^ - }$F- to increase the critical quenching concentration by a factor of 16 over the largest reported values for the Yb-doped silica. Using a custom slow-light fiber Bragg grating sensor, temperature changes up to $ - {50}\;{\rm mK}$-50mK were measured with 0.33 W/m of absorbed pump power per unit length at 1040 nm. The measured dependencies of the temperature change on the pump power and the pump wavelength are in excellent agreement with predictions from an existing model, and they reflect the fiber's groundbreaking quality for the radiation-balanced fiber lasers.

Demonstration of anti-Stokes cooling in Yb-doped ZBLAN fibers at atmospheric pressure.

For the first time, to the best of our knowledge, optical cooling is demonstrated in a fiber at atmospheric pressure. Using a specialized slow-light fiber Bragg grating temperature sensor, -5.2 mK and -0.65 K were measured in a single-mode (1% YbF3) and multimode (3% YbF3) ZBLAN fiber with respective cooling efficiencies of 2.2% and 0.90%. Fitting a recently reported quantitative model of optical cooling in fibers to the measured temperature change dependence on the pump power per unit length validates the model and allows us to infer the fibers' absorptive loss and quenching lifetime, key parameters that are scarce in literature. These values are necessary for accurate cooling predictions and will aid in the development of fibers for application in optical coolers and radiation-balanced lasers.

High-resolution slow-light fiber Bragg grating temperature sensor with phase-sensitive detection.

This Letter reports a slow-light fiber Bragg grating (FBG) temperature sensor with a record temperature resolution of ∼0.3 m°C/√Hz, a drift of only ∼1 m°C over the typical duration of a measurement (∼30 s), and negligible self-heating. This sensor is particularly useful for applications requiring the detection of very small temperature changes, such as radiation-balanced lasers and the measurement of small absorptive losses using calorimetry. The sensor performance is demonstrated by measuring the heat generated in a pumped Yb-doped fiber. The sensor is also used to measure the slow-light FBG's very weak internal absorption loss (0.02 m-1), which is found to be only ∼2% of the total loss.

Aircraft-navigation-grade laser-driven FOG with Gaussian-noise phase modulation.

A laser-driven fiber optic gyroscope (FOG) is demonstrated with an angular random walk noise of 5.5×10-4 deg/√h, a drift of 6.8×10-3 deg/h, and an inferred scale-factor stability of 0.15 ppm, making it, to the best of our knowledge, the first laser-driven FOG to satisfy the performance requirements for inertial navigation of commercial aircraft. This is achieved using Gaussian white noise phase modulation to broaden the linewidth of the source laser and to strongly suppress the narrow-linewidth optical carrier. The performance of this laser-driven FOG is shown to have better noise and only slightly higher drift than the same FOG driven by a conventional superfluorescent fiber source. This result is validated for two lasers with widely different intrinsic coherence.

Photonics sensing at the thermodynamic limit.

We report a slow-light fiber Bragg grating strain sensor with a resolution limited by the extremely low thermodynamic phase fluctuations of the fiber. This was accomplished by using a short grating (4.5 mm) to enhance the thermal phase noise, an ultra-stable interrogation laser to lower the laser frequency noise, and a slow-light mode with a high group index (∼533) to suppress all other noise sources. We demonstrate that in a similar but longer grating (21 mm), the phase noise is suppressed in inverse proportion to the square root of the length, in accordance with theory, leading to a strain resolution as low as 130 fε/√Hz and a minimum detectable length of ∼3×10-15 m at 1.5 kHz.

Pseudo-random-bit-sequence phase modulation for reduced errors in a fiber optic gyroscope.

Low noise and drift in a laser-driven fiber optic gyroscope (FOG) are demonstrated by interrogating the sensor with a low-coherence laser. The laser coherence was reduced by broadening its optical spectrum using an external electro-optic phase modulator driven by either a sinusoidal or a pseudo-random bit sequence (PRBS) waveform. The noise reduction measured in a FOG driven by a modulated laser agrees with the calculations based on the broadened laser spectrum. Using PRBS modulation, the linewidth of a laser was broadened from 10 MHz to more than 10 GHz, leading to a measured FOG noise of only 0.00073 deg/√h and a drift of 0.023 deg/h. To the best of our knowledge, these are the lowest noise and drift reported in a laser-driven FOG, and this noise is below the requirement for the inertial navigation of aircraft.

High Purcell factor in fiber Bragg gratings utilizing the fundamental slow-light mode.

We demonstrate through numerical simulations that the slow-light resonances that exist in strong, apodized fiber Bragg gratings (FBGs) fabricated with femtosecond pulses in deuterium-loaded fibers can exhibit very large intensity enhancements and Purcell factors with the proper optimization of their length. This potential is illustrated with two saturated FBGs that are less than 5 mm long and have been annealed to reduce their internal loss. The first one exhibits the largest measured Purcell factor in an all-fiber device (38.7), and the second one exhibits the largest intensity enhancement (1525). These devices are anticipated to have significant applications in quantum-dot lasers, nonlinear fiber devices, and cavity quantum-electrodynamics experiments.

Slowing down light to 300 km/s in a deuterium-loaded fiber Bragg grating.

We report light propagation with a group velocity of only 300 km/s, a group index of 1010, and a group delay of 42 ns, in a strong apodized fiber Bragg grating 12.5 mm in length. The grating was fabricated in a deuterium-loaded fiber using a femtosecond laser and a phase mask, followed by annealing to reduce residual losses. Data analysis indicates a strong index modulation of 1.98×10(-3) and an ultra-low single-pass power loss of 0.010 dB.

Fiber-feedback optical parametric oscillator for half-harmonic generation of sub-100-fs frequency combs around 2 µm.

We demonstrate a femtosecond fiber-feedback optical parametric oscillator (OPO) at degeneracy. The OPO cavity comprises an 80-cm-long fiber composed of a combination of normal and anomalous dispersion sections that provide a net intracavity group delay dispersion close to zero. By using a mode-locked, Yb-doped fiber laser as the pump, we achieved half-harmonic generation of 250-MHz, 1.2-nJ nearly transform-limited 97-fs pulses centered at 2090 nm with a total conversion efficiency of 36%.

Piconewton force measurement using a nanometric photonic crystal diaphragm.

A compact force fiber sensor capable of measuring forces at the piconewton level is reported. It consists of a miniature Fabry-Perot cavity fabricated at the tip a single-mode fiber, in which the external reflector is a compliant photonic-crystal diaphragm that deflects when subjected to a force. In the laboratory environment, this sensor was able to detect a force of only ∼4 pN generated by the radiation pressure of a laser beam. Its measured minimum detectable force (MDF) at 3 kHz was as weak as 1.3 pN/√Hz. In a quiet environment, the measured noise was ∼16 times lower, and the MDF predicted to be ∼76 fN/√Hz.

Observation of ~20 ns group delay in a low-loss apodized fiber Bragg grating.

We report a record group delay of 19.5 ns (an equivalent group index of 292) measured in a strongly apodized, 2 cm long, femtosecond fiber Bragg grating (FBG). This significant (~4-fold) improvement over the previous record results from the presence of a Fabry-Perot arising from the apodization. The measured group-index spectrum is well explained by a model that accounts for the apodized profiles of the index modulation, propagation loss, and birefringence of the grating. The peak power loss inferred from this model is only ~0.12 m⁻¹, which is one of the lowest values reported for an FBG.