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New light sources with low spatial coherence and high brightness are crucial for high‐quality imaging. In this paper, such a light source is reported, which is formed by combining random lasing used as seed light with a multimode fiber (MMF) mediated master oscillator power amplifier (MOPA) for power scaling and spatial decoherence. Experimental results show that the proposed multimode random fiber laser (MM‐RFL), with low noise, high spectral density, and high power efficiency, can achieve a maximum output power of 100.6 W and speckle contrast of as low as 0.037. Furthermore, MM‐RFL‐based speckle‐free imaging is demonstrated. The imaging quality can be improved by increasing the output power of the MM‐RFL as its spatial coherence decreases with an increasing number of effective modes excited in the MMF. Such a MM‐RFL provides a powerful light source for speckle‐free imaging applications where high power and low spatial coherence are essential for high‐quality imaging. [ABSTRACT FROM AUTHOR]

Nonlinear optical materials are key platforms in various electronic and photonic devices. This study investigated the feasibility of using bulk germanium monosulfide (GeS) particles to fabricate a saturable absorber (SA) for mode locking of fiber lasers. Bulk GeS particles were prepared via mechanical exfoliation without chemical etching. A theoretical investigation based on density functional theory (DFT) calculations was conducted to determine the origin of the saturable absorption of bulk GeS in the short-wavelength infrared (SWIR) wavelength region despite its large energy bandgap. Theoretically, the experimentally observed SWIR absorption of bulk GeS could be attributed to an energy bandgap reduction caused by strains on the bulk GeS crystal lattice structure. Using the bulk particles, an SA with a saturation power and modulation depth of approximately 38 W and 18 %, respectively, was realized at 1550 nm. The SA was then used in an erbium fiber ring cavity to produce stable optical pulses with a temporal width of ∼796 fs at ∼1556 nm. These results indicate that bulk GeS particles can be a low-cost, efficient, and nonlinear optical material for the realization of SAs in the SWIR region. • The feasibility of using bulk GeS particles to fabricate a saturable absorber was investigated. • A theoretical investigation based on DFT calculations was conducted to determine the bandgap of the bulk GeS. • The SWIR absorption of bulk GeS was theoretically found to be attributable to an energy bandgap reduction caused by strains. • Using an SA based on GeS bulk particles, 796-fs mode-locked pulses were readily produced at ∼1556 nm from a fiber laser. [ABSTRACT FROM AUTHOR]

Distributed sensing plays an important role in many industrial applications. Sensor arrays consisting of identical ultraweak fiber Bragg gratings can be applied to such scenarios by monitoring the optical spectral response of the sensors. Yet, the spatial resolution, the sampling speed, and the supported number of sensors in the array are mostly determined by the interrogation scheme. While dual-optical frequency comb techniques had been successfully demonstrated as a high-resolution spectroscopic technique in metrology, it had been beyond the reach of industrial sensing applications because of its high complexity and costs. It is proposed and demonstrated here that, enabled by a simple fiber-optic, dual-comb spectroscopy setup consisting of a fiber laser without frequency stabilization, interrogation of an ultraweak fiber sensor array can be realized. High spatial and spectral resolution as well as kilohertz interrogation speed is achieved in both static and dynamic measurements. It is expected that our depth-resolved dual-comb scheme can measure hundreds of sensors response in less than 1 ms. It is also flexible to deal with targets closely spaced or farther apart from each other. This shows the potential of applying dual-comb spectroscopy to more and more industrial application areas. [ABSTRACT FROM AUTHOR]

We demonstrated a linear-cavity fiber laser using a fiber Bragg grating (FBG) fabricated by femtosecond laser and a Sagnac loop as cavity mirrors. The temperature sensing response of the fiber laser was characterized by placing the FBG in a high-temperature environment. The stability of the fiber laser at high temperature was improved after the FBG was annealed at 1100 °C. The fiber laser can work stably as a temperature sensor at 1000 °C and its temperature sensing sensitivity is approximately 15.9 pm/°C from 300 °C to 1000 °C. • A fiber laser based on an FBG fabricated by femtosecond laser and a Sagnac loop as cavity mirrors is demonstrated. • The fiber laser can work stably as a temperature sensor at 1000 °C after the FBG was annealed at 1100 °C. • The wavelength of the fiber laser had a quadratic dependence on temperature from room temperature to 1000 °C. • The temperature–wavelength had a good linear relationship from 300 °C to 1000 °C. Its sensing sensitivity was about 15.9 pm/°C. [ABSTRACT FROM AUTHOR]

A general ${N}$ -by- ${N}$ coupling matrix that provides lossless uniform feedback to all laser array elements is demonstrated in the analysis of a passive coherent beam combining resonator. The resonator contains a spatial filter that sends rejected optical power from the combining optic back into the cavity. This recycling of otherwise unused power yields an increase in the average laser output power that scales with array size ${N}$ , even in the absence of passive phase effects that depend on the gain, intensity, and wavelength of the laser mode. To achieve this improvement, tailored path lengths and mirror reflectivities are required in the feedback arms, where the phase may be fixed, whereas in the gain arms, the path lengths vary randomly. [ABSTRACT FROM AUTHOR]

Highlights • We demonstrate a dual-wavelength and dual-pulse Yb-doped fiber with a Tm-Ho co-doped fiber as saturable absorber. • The pulse at 1064 nm is generated via SA-induced self Q-switching. • The pulse at 2127.7 nm is generated by gain-switching with the laser at 1064 nm as the pump for the Tm-Ho co-doped fiber. • The pulse duration of the laser at 1064 nm can reach up to 1.4 μs. Abstract We demonstrate a dual-wavelength and dual-pulse Yb-doped fiber laser with a Tm-Ho co-doped fiber as saturable absorber (SA). The wavelength of the first pulse in the dual-pulsing output is at 1064 nm generated via SA-induced self Q-switching and the second pulse is at 2127.7 nm and generated by gain-switching with the laser at 1064 nm as the pump for the Tm ions in the Tm-Ho co-doped fiber and thus induced stimulated emission. With the increases of the pump power, the pulse duration of the two pulses is decreased depending on the level of the pump. The pulse duration of the laser at 1064 nm decreases from 1.4 μs to 296 ns when the pump power increases from 275 mW to 475 mW, while the pulse duration at 2127.7 nm is reduced from 440 ns to 246 ns when the pump increases from 300 mW to 475 mW. Accordingly, the repetition rate is decreased from 17.4 kHz to 6.8 kHz. The experimental result shows that Tm-Ho co-doped fiber is an excellent candidate for SA-induced Q-switching at 1064 nm. [ABSTRACT FROM AUTHOR]

Highlights • Q-switched and mode-locked Tm-Ho laser with monolayer WS 2 SA by CVD method. • Mode-locked pulse-duration is up to 1.07 ns. • Q-switched pulse-duration is up to 6.2 μs. Abstract We demonstrate a passively Q-switched and mode-locked Tm-Ho co-doped fiber laser based on a WS 2 saturable absorber (SA) fabricated by chemical vapor deposition method. When the pump power increases from 0.32 W to 0.38 W, the repetition rate of the Q-switched laser pulse increases 47.1 kHz to 90.1 kHz and the pulse width decreases from 6.2 μs to 1.7 μs. In addition, the mode-locking operation can also be observed by adjusting the polarization controller (PC) and mode-locked laser pulses at 1883.2 nm can be achieved at a high pump power between 0.5 W and 0.64 W. The pulse duration is 1.07 ns and repetition rate is 23.8 MHz and the pulse is the longest mode-locked pulse with WS 2 -SA ever reported. [ABSTRACT FROM AUTHOR]

Distributed feedback Raman fiber lasers with $\pi $ -phase-shifted uniform gratings are modeled and simulated in the steady state, to optimize their performance. Using the parameters of realistic devices, it is found that the position change of the $\pi $ -phase-shift in a constant-strength uniform grating has a significant impact on the laser performance including right and left-hand-side output power and emission frequency, and linewidth. Also, it is shown that the optimum phase-shift position to maximize the laser uni-directionality is dependent on pump power and fiber loss value. A new design approach based on an engineered $\pi $ -phase-shifted step-like-strength uniform grating is presented demonstrating that even for a high-loss fiber (0.1 dB/m), the laser power and linewidth can be efficiently increased and decreased by ~25% and ~15%, respectively, for the same total device length and pumping condition, just by the proper choices of the phase-shift position and two coupling coefficients. [ABSTRACT FROM AUTHOR]

The potential of Fe 3 O 4 nanoparticles as a base material for saturable absorbers (SAs) that can operate in the 2 μm-wavelength regime, was investigated. An SA was fabricated on a sandwich-structured fiber-ferrule platform through the deposition of an Fe 3 O 4 mixture with polyvinyl alcohol (PVA) onto the end surface of the fiber ferrule. Using the prepared Fe 3 O 4 /PVA-composite-based SA within a Tm-Ho codoped fiber laser, stable Q-switched pulses were readily produced at 1894 nm. The temporal characteristics of the output pulses were investigated as a function of the pump power, and the maximum energy of the output pulses was 322 nJ. The temporal width of the output pulses was measured as 2.6 µs at the maximum pulse-repetition rate of 37.2 kHz. The output performance of the passively Q-switched Tm-Ho-codoped fiber laser in this work was compared with those of recently demonstrated, passively Q-switched Tm-doped fiber lasers for which the other types of saturable-absorption materials. [ABSTRACT FROM AUTHOR]

We experimentally demonstrate a switchable narrow linewidth single-longitudinal-mode (SLM) erbium-doped fiber (EDF) ring laser with LP 11 transverse mode output. The laser is based on a mode selective all-fiber fused coupler which is composed of a single-mode fiber (SMF) and a two-mode fiber (TMF). By controlling the polarization state of the output light, the laser can provide narrow linewidth SLM output with LP 11 transverse mode at two specific wavelengths, which correspond to two transmission peaks of the chirped moiré fiber grating (CMFBG). The 20 dB linewidth of the fiber laser for each wavelength is approximately 7.2 and 6.4 kHz. [ABSTRACT FROM AUTHOR]

• GHz pulse bursts with tunable intra-burst repetition rate and number of pulses are demonstrated experimentally and numerically based on the inherent nature of the fiber lasing cavity for the first time. • With the increase of pump power, only the number of pulses per burst increases one by one in the positive feedback region of the NPR SA, while only the number of pulses per burst increases one by one in the inverse feedback region. • The 22.8-GHz intra-burst repetition rate is the highest value in all-fiber laser systems. Optical pulse bursts are multiple pulses that arise from the balance between attractive and repulsive effects and are used for pulsed laser deposition, flow measurement, various micromachining, spectroscopic, and biomedicine. Different application fields have special requirements for the pulse burst, such as burst width, frequency, number of pulses per burst, and the pulse interval etc. Here, we propose and demonstrate the generation of GHz pulse bursts with both tunable pulse number and pulse interval in a nonlinear polarization rotation (NPR) mode-locked fiber laser for the first time. Taking advantage of the sinusoidal saturated absorption of the NPR, with the increase of pump power, only the number of pulses per burst increases one by one in a relatively low pump power range, while only the pulse interval increases gradually in a high pump power range. In addition, one pulse burst with a 22.8-GHz repetition rate is achieved, which is the highest intra-burst repetition rate directly obtained in all-fiber lasers to the best of our knowledge. The numerical results agree qualitatively well with the experiments. Our work gives an insight into the formation of pulse bursts in mode-locked fiber lasers. [ABSTRACT FROM AUTHOR]

A dynamic spectroscopic characterization scheme for fast spectral variations based on dual asynchronous undersampling with triple optical frequency combs is proposed and demonstrated here. The tri-comb scheme leverages two of the combs to simultaneously sample the fast spectral variations applied to the third comb. The repetition rate differences for both pairs of combs are relatively small, in order to maintain sufficiently large spectral measurement ranges. The corresponding sampling rates could be much slower than a fast time-varying spectral modulation to be measured. Nevertheless, despite of the frequency ambiguity in the undersampled dual-comb results, it is shown that it is possible to identify the original frequency of spectral variations based on the difference in the undersampled frequency domain information. Experimental results show that our scheme can increase the frequency demodulation range by at least two orders of magnitude while retaining the accuracy and spectral measurement range of similar DCS schemes. Demonstration based on a tri-comb mode-locked fiber laser is carried out to measure the dynamic spectral response from a fiber Bragg grating (FBG), to which up to 110 kHz of vibration is applied. This scheme could further extend the capability of dual-comb spectroscopic techniques, and it also illustrates the potential of the low-complexity multi-comb laser source for novel yet practical applications. [ABSTRACT FROM AUTHOR]

Abstract: We demonstrate an ultra-wideband wavelength-tunable actively mode-locked short-cavity laser employing a 151-cm-long bismuth-based highly nonlinear erbium-doped fiber (Bi-HNL-EDF). A wavelength tuning range of 87nm from 1533nm to 1620nm can be achieved because the Bi-HNL-EDF has an ultra-wide gain bandwidth. High nonlinearity of the Bi-HNL-EDF also collaborates with spectral filtering by an optical bandpass filter to suppress the supermode noise quite effectively. Total length of the fiber ring cavity is as short as 16m. Thus, stable and clean 5.6–6.1ps pulses with a repetition rate of 10GHz are successfully obtained over the wavelength tuning range almost completely covering both the conventional wavelength band (1530–1565nm) and the longer wavelength band (1565–1625nm). The bismuth-based short-cavity fiber laser also shows good performance in the back-to-back bit-error-rate measurements, and maintains bit-error-free mode-locking operation throughout the entire wavelength tuning range. [Copyright &y& Elsevier]

• First demonstration of self-mode-locked dark solitons in a Tm-Ho doped fiber laser. • The self-mode-locking operation is achieved by inserting a long gain fiber. • Dark solitons are generated by exploiting the reverse saturable absorption effect. • Our simulation based on the NLSE equation confirms the dark soliton generation process. • Our method shows a higher damage threshold and a record laser power of 149.6 mW. We present the first experimental demonstration of self-mode-locked dark solitons in a Tm-Ho doped fiber laser at 1.93 μm with an average power of 149.6 mW and repetition rate of 15.9 MHz. In our system, the dark solitons are generated by self-mode-locking via exploiting the reverse saturable excited-state absorption of a gain fiber, which achieves a higher damage threshold and accordingly laser power over conventional methods based on extra saturable absorber or nonlinear polarization rotation. In the experiments, we study the evolution from high-order to low-order dark solitons by adjusting the pump power, which is attributed to the excited-state absorption in Tm3+ between 3F 4 and 3H 4 energy level. Simulations have been performed to show dark soliton generation by the reverse-saturable absorption mechanism in Tm-Ho doped fiber laser based on the extended nonlinear Schrödinger equation. The results confirm our new approach can generate stable dark solitons of high average power, which may find important applications in laser sensing and laser communications. [ABSTRACT FROM AUTHOR]

Abstract: We present spectroscopic measurements and amplifier experiments on a new type of Yb-doped glass optical fiber derived from Yb:YAG crystal rods. The goal of this work is to provide a feasibility analysis of these novel silicate glasses as heavily-doped gain media for fiber-amplifier applications. Amplifier performance currently is limited by background losses arising from the precursor crystalline rod. Both basic spectroscopic measurements and fiber amplifier experiments, in addition to modeling efforts that include the background losses, are presented. The findings show that Yb:YAG-derived fibers are spectroscopically very similar to more conventional Yb-doped aluminosilicate materials but can be doped to considerably higher concentration, generally have greater doping and compositional uniformity, and possess advantageous stimulated Brillouin scattering (SBS) characteristics. [Copyright &y& Elsevier]

A distributed telemeter is herein presented, which has specifically been designed for the high-speed high-resolution monitoring of trains in railway portals but can be adapted to a number of industrial and environmental applications. The system uses a single transmitter/receiver unit to feed, using optical fiber pairs, a number of measuring stations. It makes use of an ultrashort fiber laser operating at 400 kHz or, alternatively, a fiber-coupled diode laser and presents in-house developed solutions to obtain a 40-dB dynamic range, thus allowing to monitor targets within large distance/angle/color intervals. The system has been developed, tested, and installed on a train-monitoring portal in the Roma–Napoli railway, in the framework of an effort to increase the safety of trains entering tunnels in the Italian railway system. [ABSTRACT FROM AUTHOR]

Abstract: The structure of an all-fiber Erbium-doped fiber ring cavity with bidirectional single-mode oscillations and a rotation-sensitive beat signal for gyro applications is reported in this letter. The structure is achieved by introducing a polarization splitting method to filter the bidirectional oscillations and to control the mode coupling of the counter-propagating waves. Two distinct behaviors of the beat signal are identified, namely, the linear mode coupling situation caused by the back-reflection of the optical elements and the nonlinear mode coupling situation attributed to spatially non-uniform saturation in gain medium. Experiments are conducted to show the cavity rotation rate dependence of the beat frequency shift. The results are in good agreement with the theoretical predictions. [ABSTRACT FROM AUTHOR]

Great increase in the output power of erbium-doped fluoride fiber lasers has recently been achieved, but the predicted high slope efficiency was not observed. We conduct an investigation into the possible underlying reasons, based on the newly available spectroscopic data, and our measurements on the running fiber laser are compared against computer simulation. The results indicate that interionic processes in the present fibers are much weaker than previously expected and only allow for an operation with 15–30% efficiency where the excited-state absorption is shown to have an important contribution. [ABSTRACT FROM PUBLISHER]

Continuously pumped Yb-doped double-clad fiber lasers exhibit two kinds of instabilities, known as self-sustained pulsing and self-mode locking (SML). We study numerically the role of self-phase modulation (SPM) on the onset of these stabilities by using a spatiotemporal model that takes into account fully the length dependence of both the optical gain and spontaneous emission, while also including the nonlinear effects within the fiber. Self-sustained pulsations originate from relaxation oscillations. Although they die out quickly in the absence of SPM, our results show that SPM restores these oscillations when the laser power exceeds a certain value. SML has its origin in the spontaneous-emission noise acting as a seed. The laser cavity amplifies this noise the most at frequencies associated with its longitudinal modes, resulting in periodic power variations with an amplitude < 1\% (without SPM). We find that SPM plays again a critical role and increases their amplitude to above the 10% level. We also show that the SPM-induced instabilities can be suppressed by inserting a passive fiber of suitable length inside the laser cavity, which is in agreement with recent experimental work. [ABSTRACT FROM PUBLISHER]

We theoretically detail and explain how to control unwanted excess gain for higher order modes in cladding-pumped large-core ytterbium-doped fiber amplifiers operating on the fundamental mode by using a small inner cladding and pumping close to the emission wavelength in a so-called tandem-pump configuration. Such excess gain can otherwise limit the core size. We study this experimentally by comparing the level of amplified spontaneous emission at the edge of the core in a conventional diode-pumped and tandem-pumped large-core ytterbium-doped multimode fiber. We also discuss the potential benefits of tandem pumping in reducing fiber photodarkening. [ABSTRACT FROM PUBLISHER]

A square-waveform generator based on a fiber optical parametric oscillator (FOPO) is proposed and demonstrated experimentally. The FOPO works fundamentally by extracting the longitudinal modes beating signal to drive the intracavity amplitude modulator. In order to broaden the longitudinal mode spacing, two sub-ring cavities with a short cavity length are used. The longitudinal modes are phase locked, which ensures the generated microwave signal with very low phase noise. By biasing the amplitude modulator, the even-order beating signals are suppressed, while the odd-order beating signals are enhanced. Thus, the FOPO is self-starting and can provide stable square-waveform output with the repetition rate of 1.133 GHz at the signal and idler simultaneously. It is tunable over 10 nm, which is only limited by the gain bandwidth of the fiber optical parametric amplifier of the current configuration. [ABSTRACT FROM PUBLISHER]

A theoretical and experimental analysis of the pump-noise contribution to frequency noise and linewidth in distributed-feedback fiber lasers is presented. An analytic expression for the frequency domain transfer function from pump to frequency fluctuations is derived and a formula for the pump-dependent frequency noise spectrum given. These results are confirmed experimentally for two dissimilar distributed-feedback fiber lasers with moderate Er concentration. The transition from fundamental thermal noise at low pump intensity to pump induced frequency noise at high pump intensity is experimentally observed, thus, clarifying the general nature of frequency noise in compact fiber lasers. The relationship between frequency noise and linewidth is discussed. [ABSTRACT FROM AUTHOR]

A new configuration of self-seeding scheme has been developed for wavelength-tunable optical pulse generation. In the system, an erbium-doped fiber laser is used to provide a strong wavelength seeding power and as a result, an extremely high sidemode suppression ratio of larger than 50 dB over the whole wavelength-tuning range of 13 nm can be obtained, with good pulse stability. When compared with other erbium-doped fiber amplifier configurations reported previously, the developed system exhibits higher pump efficiency and signal amplification capability. The system is simple, robust, and of low cost. [ABSTRACT FROM AUTHOR]

The configuration of a simple improved high efficiency automatic-power-controlled and gain-clamped EDFA (APC-GC-EDFA) for broadband passive optical networking systems (BPON) is presented here. In order to compensate the phase and amplitude variation due to the different distance between the optical line terminal (OLT) and optical network units (ONU), the APC-GC-EDFA need to be employed. A single 980 nm laser module is employed as the primary pump. To extend the bandwidth, all C-band ASE is recycled as the secondary pump to enhance the gain efficiency. An electrical feedback circuit is used as a multi-wavelength channel transmitter monitor for the automatic power control to improve the gain-flattened flatness for stable amplification. The experimental results prove that the EDFA system can provide flatter clamped gain in both C-band and L-band configurations. The gain flatness wavelength ranging from 1530 to 1610 nm is within 32.83 ± 0.64 dB, i.e. below 1.95 %. The gains are clamped at 33.85 ± 0.65 dB for the input signal power of −40 dBm to −10 dBm. The range of noise figure is between 6.37 and 6.56, which is slightly lower compared to that of unclamped amplifiers. This will be very useful for measuring the gain flatness of APC-GC-EDFA. Finally, we have also demonstrated the records of the overall simultaneous dynamics measurements for the new system stabilization. The carrier to noise ratio (CNR) is 49.5 to 50.8 dBc which is above the National Television System Committee (NTSC) standard of 43 dBc, and both composite second order (CSO) 69.2 to 71.5 dBc and composite triple beat (CTB) of 69.8 to 72.2 dBc are above 53 dBc. The recorded corresponding rise-time of 1.087 ms indicates that the system does not exhibit any overshoot of gain or ASE variation due to the signal at the beginning of the pulse. [ABSTRACT FROM AUTHOR]

In this study, bismuth-doped fiber lasers operating at the wavelength of 1179 nm with an optical efficiency of up to 28% are realized. The fiber gain upon 1-μmpumping declines, while the unsaturable absorption increases with increasing the small-signal absorption. We conclude that up-conversion and excited-state absorption are responsible for limiting the efficiency of such lasers. [ABSTRACT FROM AUTHOR]

Using the optical fibers and ultrashort pulses, we can demonstrate highly functional optical control. We have succeeded in generation of low-noise, ultraflat, high-quality supercontinuum (SC). Recently, octave spanning high-quality SC is also generated. The carrier envelope offset frequency measurement and ultrahigh resolution optical coherence tomography are demonstrated using SC. We can also generate widely wavelength-tunable ultrashort soliton pulses using soliton self-frequency shift (SSFS). So far, several new techniques have been proposed and demonstrated using SSFS. The author has also discovered novel two-pulse trapping phenomena. The ultrafast all-optical switching is demonstrated using the pulse trapping techniques, and principles and characteristics are described. [ABSTRACT FROM AUTHOR]

The reciprocity relation of McCumber which states that the emission and absorption spectra of broad solid-state laser transitions are related by the classical Boltzmann factor is critically examined. It is argued that it should be treated as an exact property of homogeneously broadened amorphous host gain media at room temperature. The contradiction between these conclusions and those of recent literature arguing against the applicability of the McCumber relation for erbium doped silica is resolved. Experimental support for the McCumber relationship in erbium doped silica fiber is provided. [ABSTRACT FROM AUTHOR]

We presented a three-dimensional optical memory using a newly developed compact fiber laser. The fiber laser can produce pulse with high peak power of 5.3 kW. This allows two-photon absorption recording for next generation optical recording media possible for consumer application. We demonstrated two-photon absorption recording on a photochromic materials; diarylethene and achieved multilayer recording using the developed fiber laser. [ABSTRACT FROM AUTHOR]

A fiber-based frequency comb has been developed to measure the frequency of a 633-nm iodine-stabilized laser at the National Metrology Institute of Japan (NMIJ). The measured frequency was consistent with the previous results measured with a Ti:sapphire-based comb. The NMLJ comb was shipped to Australia for the validation of the measurement capability of the National Measurement Institute, Australia (NMIA) and NMIJ combs using a common microwave reference and a common optical frequency. Consequently, the frequency consistency of the two combs was approximately 8 × 10-17. Furthermore, we demonstrate that an absolute mode number of the comb can easily and clearly be determined by using the two combs. [ABSTRACT FROM AUTHOR]

We conducted an investigation on the saturable absorption properties of Ti 2 AlN through both experimental material characterizations and a density functional theory energy band calculation. On the basis of the saturable absorption properties of the Ti 2 AlN MAX phase that was observed in this investigation, a fiberized mode-locker operable in the 1.5-μm spectral region was fabricated with a side polished fiber platform. The modulation depth and saturation intensity of the mode-locker were measured as ~7% and ~91 MW/cm2, respectively. By inserting the mode-locker into an Er3+ doped fiber ring cavity, conventional soliton pulses, which were stably mode-locked, could be generated at ~1557.6 nm. [ABSTRACT FROM AUTHOR]

• This review summarizes the application of fiber lasers in the field of non-invasive blood glucose monitoring. • This review focuses on five optical methods that have received the most attention, all of which can use fiber lasers as light sources. • The structure and composition of the fiber laser, pulse output characteristics and how to apply it to non-invasive blood glucose monitoring are described in detail. • The blood glucose monitoring methods are classified, and the basic theory, advantages and future development of non-invasive blood glucose monitoring are discussed. • The work process, data processing methods, methods of evaluating system performance, and methods of improving system performance of the non-invasive blood glucose monitoring system are introduced. Diabetes is a chronic metabolic disorder, which may lead to a series of serious complications and endanger human life in severe cases. Regular monitoring of blood sugar levels can help patients prevent diabetes complications. In this review, firstly, the causes and classification of diabetes, the background knowledge of non-invasive blood glucose monitoring, the classification of blood glucose monitoring methods, the basic theory of optical methods, and the process of non-invasive blood glucose monitoring are described. Then, the basic structure of the fiber laser, and the significant characteristics of the output light pulse are introduced. Most importantly, five optical methods that the fiber laser can be applied to non-invasive blood glucose monitoring are described in detail. Finally, this review describes the process of electrical signal processing in the non-invasive blood glucose monitoring device, some important algorithms, and the means of device performance evaluation. [ABSTRACT FROM AUTHOR]

Time-domain magnetization dynamics of sputtered GdFeCo (30 nm) amorphous alloy films was measured by pump-probe method using high-power ultra-short pulse fiber laser. The effective g-factor geff and effective damping constant αeff of the GdFeCo films were estimated by using a numerical calculation of Landau-Lifshitz-Gilbert equation. The precessional frequency took a maximum near the magnetization compensation composition CM of the GdFeCo, while the estimated geff and αeff increased around the angular momentum compensation composition CA. The compositional dependences of geff and αeff were roughly described by a mean-field model. The geff and αeff were also estimated from the ferromagnetic resonance (FMR) spectra, and the data from the FMR spectra agreed well with those from the pump-probe measurement except for the composition near CM. The FMR method was unable to excite the magnetization near CM because of the small net magnetization. [ABSTRACT FROM AUTHOR]

Bismuth-doped fiber lasers with efficiency of up to 32% at room temperature and up to 52% at 77 K have been developed. The dependence of the efficiency on the pump power and the pump wavelength is interpreted on the basis of spectroscopic measurements. [ABSTRACT FROM AUTHOR]

We demonstrate an RF spectrum analyzer based on spectral-hole burning (SHB) that operates with unity probability of intercept and resolution under 100 kHz. An SHB crystal, which consists of rare-earth ions doped into a crystal host, records the power spectrum of an RF signal modulated onto an optical carrier as a series of spectral holes that persist for about 10 ms. While the crystal's homogeneous and inhomogeneous linewidths place the fundamental limits on resolution and bandwidth, respectively, the practical limits depend on the lasers used to interrogate the record stored in the crystal's absorption profile. Up to now, SHB spectrum analyzers have used chirped beams from externally modulated, stabilized lasers, which have linewidths of under 10 kHz but cannot chirp over much more than octave bandwidths, or directly modulated diode lasers, which can chirp over more than 20 GHz but have linewidths of about ! MHz. Switching to chirped fiber lasers, which have natural linewidths of under 2 kHz and chirping linewidths on the order of 10 kHz, produces a measurement with fine resolution without any laser stabilization. In addition, by chirping the fiber laser with a sufficiently fast piezo, the resulting chirp could extend over tens of gigahertz in under 10 ms, yielding both fine resolution and broad bandwidth without extraordinary stabilization schemes. [ABSTRACT FROM AUTHOR]

A numerical model for multiwavelength actively mode-locked erbium-doped fiber lasers (AML-EDFL) is presented. Using the model, the influences of four-wave mixing (FWM) and phase modulation effects (i.e., self-phase modulation (SPM) and cross-phase modulation (XPM)) can be studied, respectively. We use it to investigate AML-EDFLs incorporating a highly nonlinear fiber (HNLF) and explore the mechanisms for gain competition suppression (GCS). The net dispersion of the cavity is found critical for GCS. In the small dispersion regime, FWM dominates GCS, but SPM and XPM deteriorate the output pulses. In the large anomalous dispersion regime, SPM and XPM, however, enhance GCS and produce high-quality pulses. Degree of GCS is also quantitatively evaluated. The simulation results are at last confirmed by experiments. [ABSTRACT FROM AUTHOR]

We present a new modeling method for cladding- pumped highly Er-Yb co-doped fiber amplifiers. The method combines the beam propagation method (BPM) for the field propagation and nonlinear rate equations for the laser medium. The pump absorption, which is obtained numerically from the nonlinear rate equations, is used in BPM calculations of the field propagation. As a result, our method can effectively deal with difficult issues such as the multimode nature of cladding pumped schemes even in complicated structures, as well as strong nonlin-earities in highly Er-Yb-doped fiber amplifiers. The simulation results are in very good agreement with experiments having different structures and different concentrations of Er-Yb. The method is particularly well suited to active fibers with low amplified spontaneous emission power, but it can also be developed further for more general cases. [ABSTRACT FROM AUTHOR]

This paper presents an improved curvature loss formula for optical waveguides, which is shown to accurately predict the bend loss of both single-mode and multimode fibers. The formula expands upon a previous formula derived by Marcuse, greatly improving its accuracy for the case of multimode fiber. Also presented are the results of bent fiber simulations using the beam propagation method (BPM), and experimental measurements of bend loss. Agreement among simulation, formula and measurement support the validity of both theoretical methods. BPM simulations showed that the lowest order modes of the bent fiber were reduced to their linearly polarized constituents prior to the onset of significant bend loss. This implies that certain LP mode orientations should propagate with much lower loss than previously expected, and should impact the mode stripping ability of bent large mode area fibers, as employed in fiber lasers and amplifiers. [ABSTRACT FROM AUTHOR]

A detailed analysis of fundamental thermal frequency noise in distributed feedback fiber lasers (DFB FL) is presented. An expression relating the laser frequency noise spectrum to the spectrum of thermal (temperature) fluctuations in the fiber, accounting for the nontrivial DFB FL mode shape, is derived. A new expression for the thermal fluctuation spectrum of single-mode optical fiber is derived which predicts high-frequency thermal noise 3 dB higher than previously published results. Theoretical predictions are compared against experimental measurements for a well characterized DFB FL. Anomalous 1 / f noise at low frequency is discussed. [ABSTRACT FROM AUTHOR]

A new laser architecture emitting alternate multi-wavelength picosecond pulses at room temperature is proposed. The generation is based on a single active component, an unbalanced Mach-Zehnder interferometer, inserted in an actively mode-locked erbium-doped fiber ring laser to provide both intensity modulation and tunable filtering. Control of the emission is reached by inserting an additional modulator and a periodic filter in the cavity. We provide general insight into the operation of this type of laser by examining different regimes in simulations and experiments. Using this laser platform, we experimentally demonstrate the generation of three pulse trains at three different wavelengths at repetition rates of several gigahertz. [ABSTRACT FROM AUTHOR]

Abstract: A robust, self-starting picosecond pulse source based on ytterbium (Yb3+) doped fiber laser is described. Utilizing a chirped-fiber-Bragg-grating (C-FBG) for dispersion control, solitary mode-locking is obtained without bulk dispersion compensation elements. A semiconductor saturable absorber (SESAM) is used for stable self-starting. 3.6ps pulses are produced, with 45MHz basic repetition-rate and mW scale average output power at 1060nm. Detailed numerical simulations based on the modified nonlinear Schrödinger equation agree well with the experimental results and are used as a design tool for the solitary mode-locked picosecond laser. The presented design can be simply employed in an all-fiber environmentally-stable system. [Copyright &y& Elsevier]

Abstract: We experimentally demonstrate a practical benefit of our proposed continuous-wave (CW) supercontinuum (SC) generating ring laser scheme in terms of low-cost device implementation and then investigate its output intensity noise property, as a further study to our previous paper of Ref. [J.H. Lee, Y. Takushima, K. Kikuchi, Opt. Lett. 30 (2005) 2599]. First, we implement a practical and low-cost CW SC laser using a double clad Er/Yb codoped fiber and a highly nonlinear dispersion-shifted fiber (HNL-DSF). Unlike the scheme in Ref. [J.H. Lee, Y. Takushima, K. Kikuchi, Opt. Lett. 30 (2005) 2599] the gain medium of an Er/Yb fiber is clad-pumped by low quality multimode pump LDs to underline the fact that our proposed scheme does not require any high power single-mode laser pump. A SC of a bandwidth larger than 470nm is readily achieved. Next, we measure relative-intensity-noise (RIN) of the generated SC and compare it with that of a pumped Er/Yb fiber ASE. The SC laser is found to have a much higher RIN than the ASE due to the nonlinear amplification of quantum fluctuations both in the seed light oscillation and in the Raman scattering process. [Copyright &y& Elsevier]

Abstract: We propose the combination of low- and high-birefringence fibers (Low-Bi and Hi-Bi, respectively) to form a hybrid birefringence resonator. The transmission characteristics as a function of different parameters for the fibers used within these hybrid resonators are obtained by means of Jones matrix analysis. This yields information on fiber laser features such as polarization mode-beating frequencies (PMB), which are used as readout for polarimetric sensors. We compare theoretical predictions with experimental results on the dependence of PMB frequencies as a function of wavelength and effective birefringence of the laser cavity. Applications of this hybrid fiber laser arrangement for polarimetric sensors and wavelength monitoring through PMB frequencies are also discussed. The results provide a simple way for optimizing fiber laser parameters and increase the sensitivity of PMB signals to wavelength variations. [Copyright &y& Elsevier]

A simple continuously wavelength-tunable fiber ring laser pulse source is demonstrated. Such a source employs a gain-switched multiple quantum-well Fabry-Perot laser diode in a self-seeding scheme and the output pulse wavelength can be tuned continuously via a tunable optical filter. The optical pulses generated exhibit a pulsewidth of 92 ps at the frequency of 2.6 GHz. Within a wide wavelength tuning range of 46 nm, the side-mode suppression ratio and the average power of the output optical pulses obtained are higher than 50.1 dB and 3.3 dBm, respectively, and the pulse spectral stability can be maintained in the whole wavelength tuning range. [ABSTRACT FROM AUTHOR]

We present an all-fiber system for generating pedestal-free 22-fs ultrashort pulses with a single-peak spectrum. High-power Raman soliton pulses and a normally dispersive highly nonlinear fiber are used to generate a smooth, broadened single-peak spectrum. Then, the higher order dispersion is compensated for using a hybrid fiber composed of a reverse-dispersion fiber and a standard single-mode fiber, which allows pedestal-free ultrashort pulses to be successfully generated. To our knowledge, this is the first demonstration of pedestal-free 20-fs-class ultrashort pulse generation without spectral distortion using an all-fiber configuration. [ABSTRACT FROM AUTHOR]

The output of a femtosecond fiber laser will form a frequency comb that can be phase-locked through feedback to the cavity length and pump power. A perturbative theory is developed to describe this frequency comb output, in particular for a solitonic fiber laser. The effects of resonant dispersion, saturation of the self-amplitude modulation, cavity loss, third-order dispersion, Raman scattering, self-phase modulation, and self-steepening on the spacing and offset of the fiber-laser frequency comb are given. The mechanisms by which the pump power, cavity length and cavity loss control the frequency comb spacing and offset are identified. Transfer functions are derived for the comb response to change in cavity length, pump power or cavity loss. This theory can potentially be applied to other frequency comb sources as well. [ABSTRACT FROM AUTHOR]

Abstract-We theoretically study the recovery of a harmonically actively mode-locked soliton fiber laser from pulse dropout. In such lasers, a large number of pulses propagate simultaneously in the cavity. In order to obtain stable operation, pulses that are dropped due to changes in environmental conditions should recover, while other pulses that propagate in the cavity should remain stable. Soliton perturbation theory is used to find stability conditions for the noise in a time slot where a steady state pulse exists and in a time slot where a pulse is dropped. In the stable operating region of the laser, noise is stable in the presence of a pulse while noise becomes unstable in time slots where a pulse is dropped. Such a stability condition ensures that the laser can recover from accidental pulse dropout. A good agreement between the results of a reduced model and the results of a comprehensive numerical simulation was obtained. The results of this paper may enable to improve the stability of actively mode-locked fiber lasers. [ABSTRACT FROM AUTHOR]

Using an optically driven nonlinear semiconductor mirror, we have synchronized a 1.56-pm mode-locked erbium fiber laser to the pulse train from a mode-locked ytterbium fiber laser operating at 1.05 pm. We demonstrate that a robust active or passive mode-locked picosecond pulse operation and tight control of the repetition rate can be obtained resulting in a large value of the locking tolerance for the slave laser cavity mismatch exceeding 250 μm. [ABSTRACT FROM AUTHOR]

Two models are presented to describe the complex dynamics of Q-switched clad-pumped Er3+-Yb3+ codoped fiber lasers (EYDFLs): the full model that includes the full wave- length-dependence of parameters in the 1550- and the 1080-nm band due to Er3+ and Yb3+, respectively, and the simplified model that assumes single wavelengths for both bands. The models are based on detailed rate equations that include energy transfer between Er3+ and Yb3+ ions, and spontaneous emission due to both Er3+ and Yb3+ . The simplified model is shown to agree with the full model within 10% difference. Experiments are performed using a 4-m-long single-mode clad-pumped EYDFL, and pulse energy of 14.5 µJ is achieved. Excellent agreement is shown between the simplified model and experiments. The model is further used to study the extractable energy of the laser and show limiting factors: amplified spontaneous emission (ASE) and energy storage in Yb3+ ions. The model is also used to optimize the pulse energy in terms of fiber length, pump power, and Er3+ -Yb3+ concentrations. [ABSTRACT FROM AUTHOR]

Molecular vibration-rotation transitions in overtone or combination bands are well suited as absolute wavelength standards throughout the wavelength range used for optical communication. In this paper, we describe two different implementations. In the first, radiation from a superluminescent diode is sent through a cell containing H13CN. This provides the spectrum with a characteristic signature from absorption in the 2v3 overtone band, and allows for calibration in the 1530-1560 nm range. In the second implementation, radiation from a distributed feedback (DFB) fiber laser is locked to a Doppler broadened absorption line at 1548.8 193 nm in the v1 + v3 combination band of 13C2 H2. The resulting standard has a relative Allan deviation of less than 10-8 beyond 100 s of integration time, and the signal can be disseminated through optical networks without degradation in stability. A novel feature of the fiber laser is all thermal control of the wavelength through resistive heating of a thin-film gold coating. [ABSTRACT FROM AUTHOR]