Tunable Ytterbium-Doped Fiber Laser Based on a Mechanically Induced Long Period Holey Fiber Grating

We describe a tunable double-clad Yb-doped fiber laser based on a long period fiber grating mechanically induced in a section of single mode holey fiber inserted into the laser cavity. The mechanically induced long period holey fiber grating acts as a wavelength-selective fiber filter whose central wavelength, linewidth, and strength can be tuned by changing the period, the length of the grating, and the applied pressure. The fiber laser gives a ∼12.6 nm tuning range, from ∼1079:4–1092nm, with slope efficiencies of 18.7–26.3% at this wavelength range, with respect to the launched pump power.     

Wavelength modulated off-axis integrated cavity output spectroscopy in the near infrared

A novel instrument based on an improved off-axis alignment of integrated cavity output spectroscopy (OA-ICOS) in conjunction with a wavelength modulation (WM) technique, was developed using a DFB diode laser operating in the near infrared at 1.573 μm (6357.3 cm^-1). The laser-based sensor employed a 44 cm optical cavity that provided an effective absorption path length of ∼68 m. A minimum detectable absorption of approximately 3.6 ppmv Hz^-1/2 or 2.3×10^-7 Hz^-1/2 per optical pass was obtained using second harmonic detection. We demonstrated that by implementation of the WM technique to OA-ICOS in the near infrared, the detection sensitivity was improved by a factor of 14 compared to that obtained with OA-ICOS. Measurements of CO₂ mixing ratios in ambient air have been performed by using both OA-ICOS and WM-OA-ICOS techniques for performance comparison. PACS 42.62.Fi; 07.07.Df; 33.20.Ea     

Widely-wavelength-tunable few-cycle optical pulse generation from an all-fiber erbium-doped laser system

A scheme for the construction of fiber laser systems for the generation of tunable ultrashort optical pulses is proposed. The scheme is based on the self-Raman shift of the soliton frequency in dispersion-decreasing fibers with the subsequent spectral broadening owing to the supercontinuum generation in a short highly nonlinear fiber and the compression in the corresponding fiber compressor. An all-fiber laser system for the generation of ultrashort laser pulses in the wavelength range 1.6–2.0 μm is experimentally demonstrated. In particular, the shortest pulses with a duration of 24 fs are generated at wavelengths of 1.8–1.9 μm, which corresponds to less than four optical cycles.     

Wavelength-agile laser system based on soliton self-shift and its application for broadband spectroscopy

A novel laser system for rapid-wavelength scanning applications was developed. The wavelength-tuning mechanism is the soliton self-shift of femtosecond pulses in optical fiber. Increased coupling into the fiber causes an increased wavelength red-shift within the fiber. By varying the coupling efficiency, we can generate rapid wavelength scans. Here, we demonstrate tuning between 1.665 and 1.820 m at 40 kHz repetition rate with 8 mW average output power. We applied this source to measure the overtone of the C–H stretch in gaseous butane.Drawbacks, potential improvements of the system and possible applications in other fields are discussed. PACS 42.55.WD; 42.65.Dr, 07.07.Df     

Yb-doped strictly all-fiber laser actively Q-switched by intermodal acousto-optic modulation

We show an actively Q-switched ytterbium-doped strictly all-fiber laser. Cavity loss modulation is achieved in a tapered optical fiber by core-to-cladding mode-coupling induced by travelling flexural acoustic waves. When the acoustical signal is switched-off, the optical power losses within the cavity are reduced, and then a laser pulse is emitted. Trains of Q-switched pulses were successfully obtained at repetition rates in the range 1–10 kHz, with pump powers between 59 and 88 mW, at the optical wavelength of 1064.1 nm. Best results were for laser pulses of 118 mW peak power, 1.8 μs of time width, with a pump power of 79 mW, at 7 kHz repetition rate.     

Femtosecond fiber laser based methane optical clock

An optical clock based on an Er³⁺ fiber femtosecond laser and a two-mode He–Ne/CH₄ optical frequency standard (λ=3.39 μm) is realized. Difference-frequency generation is used to down convert the 1.5-μm frequency comb of the Er³⁺ femtosecond laser to the 3.4-μm range. The generated infrared comb overlaps with the He–Ne/CH₄ laser wavelength and does not depend on the carrier–envelope offset frequency of the 1.5-μm comb. In this way a direct phase-coherent connection between the optical frequency of the He–Ne/CH₄ standard and the radio frequency pulse repetition rate of the fiber laser is established. The stability of the optical clock is measured against a commercial hydrogen maser. The measured relative instability is 1×10⁻¹² at 1 s and for averaging times less than 50 s it is determined by the microwave standard, while for longer times a drift of the He–Ne/CH₄ optical standard is dominant.     

A linear cavity multiwavelength fiber laser with adjustable lasing line number for fixed spectral regions

We report an experimental demonstration of muliwavelength erbium-doped fiber laser with adjustable wavelength number based on a power-symmetric nonlinear optical loop mirror (NOLM) in a linear cavity. The intensity-dependent loss (IDL) induced by the NOLM is used to suppress the mode competition and realize the stable multiwavelength oscillation. The controlling of the wavelength number is achieved by adjusting the strength of IDL, which is dependent on the pump power. As the pump power increases from 40 to 408 mW, 1–7 lasing line(s) at fixed wavelength around 1601 nm are obtained. The output power stability is also investigated. The most power fluctuation of single wavelength is less than 0.9 dB, when the wavelength number is increased from 1–7.     

Fiber optic evanescent field sensor for detection of explosives and CO<Subscript>2</Subscript> dissolved in water

A fiber optic approach for the determination of the carbon dioxide concentration in the gas or fluid phase during sequestration, as well as for the sensing of the explosive TNT is described. The sensor consists of a quartz glass multimode fiber with core diameter of 200 μm and is based on the evanescent field principle. Cladding and jacket of the fiber are removed in the sensing portion, therefore interaction between light within the fiber and the surrounding medium is possible. A single-mode distributed feedback (DFB) laser diode with an emission wavelength around λ= 1.57 μm and a frequency doubled passively Q-switched Cr⁴⁺:Nd³⁺:YAG microchip laser (λ= 1064 nm)are used as light sources. The experimental setup and the sensitivity of the evanescent field sensor are characterized. PACS 42.62.Fi; 42.79.Wc; 07.07.Df     

Multimode optical fiber demultiplexer-star coupler using a single gradient-index-rod lens and a multi-facet grating

An optical fiber multi-function device consisting of a single gradient-index-rod lens and a multi-facet blazed reflection grating is proposed to simultaneously realize functions of wavelength demultiplexing and optical signal distribution in a multimode optical fiber transmission system. We analyzed the demultiplexing characteristics and the tolerance of optical components using the ray trace method. This device can realize not only low loss optical signal distribution but also offers improved demultiplexing characteristics in comparison with the previously proposed demultiplexer-multiposition switch. The following characteristics are expected from the design using commercially available optical components: a working band of 0.64–0.88 μm, channel separation of 34–36 nm, 3 dB bandwidth of 27–28 nm, channel cross-talk of less than - 40 dB and minimum excess insertion loss of 0.9–2.1 dB.     

Sub-ppbv nitric oxide concentration measurements using cw thermoelectrically cooled quantum cascade laser-based integrated cavity output spectroscopy

A nitric oxide (NO) gas sensor based on a thermoelectrically cooled, continuous-wave, distributed feedback quantum cascade laser operating at 5.45 μm (1835 cm^-1) and off-axis integrated cavity output spectroscopy combined with a wavelength-modulation technique was developed to determine NO concentrations at the sub-ppbv levels that are essential for a number of applications, such as medical diagnostics, environmental monitoring, and industrial process control. The sensor employs a 50-cm-long high-finesse optical cavity that provides an effective path length of ∼700 m. A noise equivalent (SNR=1) minimum detection limit of 0.7 ppbv with a 1-s observation time was achieved. PACS 07.07.Df; 33.20.Ea; 42.62.Fi; 87.64.Je     

Laser ignition of flammable mixtures via a solid core optical fiber

To date no commercial fiber coupled laser systems have reached the irradiance and pulse energy required for flammable mixtures ignition. In this work we report preliminary results on the ignition of two-phase mixtures promoted by a laser delivering pulses through optical fiber. Experiments undertaken on free beam path configurations have allowed identification of the optical parameters required for laser ignition. The fiber coupled system used is based on a Q-switched nanosecond laser operating at 1064 nm. The fiber input angle and the focal length have been identified as the most important parameters. We demonstrated the possibility of delivering nanosecond pulses of 30 mJ focused onto a spot of 200 μm through a solid core optical fiber, and to promote ignition of n-heptane/air and JP4/air mixtures. PACS 42.62.-b; 42.81.-i; 47.55.-t; 82.33.Vx     

Thermally tunable microfiber knot resonator based erbium-doped fiber laser

A compact and tunable erbium-doped fiber laser is demonstrated using a highly doped fiber and a microfiber knot resonator (MKR) structure which is laid on the surface of a small peltier. The MKR functions as both a reflector and a tunable filter where tunability is achieved by varying the temperature of the resonator by heating the peltier. A stable laser output is achieved at the 1533 nm region with an optical signal to noise ratio (OSNR) of 27 dB using a 65 mW of 980 nm pump power. The operating wavelength of the laser can be tuned from 1532.60 nm to 1533.49 nm as the temperature is increased from the room temperature of 24 to 90 °C. It is observed that the operating wavelength shifts to a longer wavelength as the temperature increases with an efficiency of 12.4 pm/°C. This is due to the thermally induced optical phase shift attributable to the changes in effective refractive index and optical path length of the MKR loop.     

Direct measurement of group dispersion of optical components using white-light spectral interferometry

We present a simple white-light spectral interferometric technique employing a low-resolution spectrometer for a direct measurement of the group dispersion of optical components over a wide wavelength range. The technique utilizes an unbalanced Mach-Zehnder interferometer with a component under test inserted in one arm and the other arm with adjustable path length. We record a series of spectral interferograms to measure the equalization wavelength as a function of the path length difference. We measure the absolute group refractive index as a function of wavelength for a quartz crystal of known thickness and the relative one for optical fiber. In the latter case we use a microscope objective in front and a lens behind the fiber and subtract their group dispersion, which is measured by a technique of tandem interferometry including also a Michelson interferometer.     

Nonadiabatic nondegenerate excitation by optical near-field and its application to optical pulse-shape measurement

Using DCM dye grains and light of different wavelengths generated by two CW laser diodes that oscillate in the near-infrared wavelength region, visible light emission from dye grains due to near-infrared excitation based on a nonadiabatic, nondegenerate excitation process was observed for the first time. Unlike sum-frequency generation with nonlinear polarization, the difference in polarization angles of the two beams did not affect the emitted light intensity. Optical sampling based on this nonadiabatic, nondegenerate excitation principle was demonstrated for the first time. The optical pulse shape in the wavelength band of λ=1250–1350 nm, which is close to the wavelength range used for optical fiber communications, was measured with a temporal resolution of 0.8–1.1 ps.     

Robust and precise length stabilization of a 25-km long optical fiber using an optical interferometric method with a digital phase-frequency discriminator

We have developed an optical fiber length stabilization system for the distribution of reference millimeter wave signals in a long-baseline phased-array radio telescope. The fiber length was compared with an absolute wavelength reference laser using a Michelson interferometer. We used a digital servo system including a digital phase-frequency discriminator with a wide phase dynamic range and a digital signal processor (DSP) for the digital servo system. All-digital servo system made it possible to realize a robust and precise length stabilization of a 25-km long optical fiber. PACS 42.62.Eh; 42.81.Uv; 95.55.Jz     

A stable dual-wavelength fiber laser with tunable wavelength spacing using a polarization-maintaining linear cavity

We propose and experimentally demonstrate a stable dual-wavelength erbium-doped polarization-maintaining (PM) fiber laser with tunable wavelength spacing using an all-PM linear cavity that makes use of two reflection peaks from the PM fiber Bragg grating (PM-FBG). Experimental results show stable dual lasing lines with a wavelength separation of ∼0.22 nm and a large optical signal-to-noise ratio (OSNR) of over 40 dB under room temperature. By applying axial strain to the PM-FBG, the center wavelengths of the two lasing lines can be tuned over several nanometers and the wavelength separation between the lasing lines can also be tuned to as small as 0.05 nm, which, to our knowledge, is the smallest wavelength spacing ever obtained from a stable room-temperature dual-wavelength fiber laser. The proposed laser configuration has the advantages of simple structure, low loss, stable dual-wavelength operation and a very small lasing linewidth of ∼5 kHz . PACS 42.55.Wd; 42.81.-i; 42.81.Gs     

Nanometer-scale displacement sensing using self-mixing interferometry with a correlation-based signal processing technique

A self-mixing interferometer is proposed to measure nanometre-scale optical path length changes in the interferometer’s external cavity. As light source, the developed technique uses a blue emitting GaN laser diode. An external reflector, a silicon mirror, driven by a piezo nanopositioner is used to produce an interference signal which is detected with the monitor photodiode of the laser diode. Changing the optical path length of the external cavity introduces a phase difference to the interference signal. This phase difference is detected using a signal processing algorithm based on Pearson’s correlation coefficient and cubic spline interpolation techniques. The results show that the average deviation between the measured and actual displacements of the silicon mirror is 3.1 nm in the 0–110 nm displacement range. Moreover, the measured displacements follow linearly the actual displacement of the silicon mirror. Finally, the paper considers the effects produced by the temperature and current stability of the laser diode as well as dispersion effects in the external cavity of the interferometer. These reduce the sensor’s measurement accuracy especially in long-term measurements.     

光纤光栅梳状滤波器及其在新型可选波长激光器中的应用

对现有在同～段光纤上多次写入光栅制成光栅梳状滤波器的方法进行了改进,利用增加光栅长度,精确控制不同光栅的曝光时间,并结合调整光栅实际写入长度的方法,制成了不均衡度小于5％、最低峰值反射率高于95％的光纤光栅梳状滤波器;同时提出了一种新颖的可选波长光纤激光器结构。采用由上述方法制作出的反射率均衡的光纤光栅梳状滤波器和可调光滤波器结合在一起进行波长选择,实现了波长的快速准确选择,同时各个波长激光的输出特性稳定,功率均衡性好,边模抑制比大于50dB。     

Generation of optical waveforms in 1.3-μm SOA-based fiber lasers

Passive optical waveform generation is obtained in fiber lasers using a 1.3-μm semiconductor optical amplifier (SOA) as the gain medium. Various waveforms, including square wave, staircase wave, triangular wave, pulse, and dark pulse are generated in SOA-based fiber lasers by adjusting intracavity polarization controllers. The passive waveform generation might be attributed to the SOA gain dynamics and the enhanced nonlinear interaction at the 1.3-μm zero dispersion wavelength of traditional single-mode fiber (SMF), as well as the interference effect between the two sub-cavities of fiber laser. With figure-8 cavity configuration, 1250th-order harmonic pulses have been successfully demonstrated. We have also obtained a free-running SOA-based fiber laser with 3-dB spectral width of 16 nm, and the center wavelength can be tuned over 45 nm range.     

Ultra-high-precision mid-IR spectrometer II: system description and spectroscopic performance

The development and spectroscopic performance evaluation of an ultra-sensitive, mid-IR spectrometer is reported. The laser system is based upon difference-frequency generation (DFG) at ～3.5 μm by mixing a DFB diode laser at 1562 nm and a DFB fiber laser at 1083 nm using a periodically poled LiNbO₃ crystal. DFG radiation was coupled to a 100?m optical path length astigmatic Herriott cell. Sensitive and selective spectroscopic detection of formaldehyde was performed with second-harmonic detection using Peltier-cooled HgCdTe detectors. By applying computer lock-ins, dual-beam optical noise subtraction, focus matching, thermal stabilization, active wavelength control, and advanced signal processing a sensitivity corresponding to an absorbance ～1.6×10^-7 is achieved for 260 s of averaging.