We achieved the first demonstration of a wavelength-tunable mode-locked fiber laser in the L-band using all-Polarization-Maintaining (all-PM) Nonlinear Polarization Rotation (NPR). The all-PM configured laser features excellent repeatability and reliability. By increasing the pump power from 82.5 mW to 135 mW, a center wavelength-tuning from 1576.2 nm to 1592.2 nm is obtained. This non-mechanical tuning mechanism opens new possibilities for L-band wavelength-tunable lasers and their applications.
Free-space optical communication is a line-of-sight wireless communication scheme, which is preferred for its number of prime advantages over radio frequency wireless communication, such as no spectrum licensing, large bandwidth, inherent security, electromagnetic compatibility/electromagnetic interference immunity etc. Moreover, free-space optical communication also benefits from low-cost installation and maintenance. It has been studied for the next generation access networks, inter-building connections, ground-to-unmanned aerial vehicle links, underwater communication applications, inter-satellite links, deep space links etc. Among various detection approaches utilized in free-space optical communication, coherent detection can achieve the best sensitivity in a bandwidth-limited condition, effectively demodulate optical multilevel coded signals to attain high spectral efficiency, offer excellent background noise rejection. However, such an attractive free-space optical communication suffer from waveform distortion, scintillation, phase fluctuations etc. after transmission in atmospheric channels. Its link losses are almost dependent on atmospheric effects and climatic conditions. In this article, we present an up-to-date survey on coherent free-space optical communication, the atmospheric turbulent effects especially the impacts of turbulence in free-space optical links, and countermeasures against such impairments.
Real-time sensing for gas, liquid, and fine particulate matter are challenging to optical fibers since the probe light cannot interact with the molecules or particle directly. One approach of extending fiber based system to gas or fine particulate matter sensing is taking the advantage of evanescent field that is generated around a microfiber. We report our progress in fluoride microfiber environmental sensor based on the absorption or scattering properties of the generated evanescent field.
Amplitude-modulated continuous-wave laser scanner with focusing optics can realize extremely high-precision 3D measurement. Since amplitude-modulated continuous-wave scheme employs periodical modulation, the longitudinal resolution and the maximum unambiguous range are in a trade-off. Our system utilizes dual-frequency modulation compromise such trade-off. However, such an attractive laser scanner suffers from ranging ambiguity due to aliasing, which is the systematic error inherent in amplitude-modulated continuous-wave scheme. We have removed the ranging ambiguity by aliasing synthesis. Secondly, the acquired 3D point clouds contain phase jumping at the maximum unambiguous range. With leveraging the relationship between the intensity and spatial information, the phase jumping was unwrapped to recover the spatial continuity. Thirdly, the 3D point clouds in the defocused region of the amplitude-modulated continuous-wave laser scanner distort since the depth-of-focus of focusing optics is generally cm order. The 3D point clouds in the defocused area are contaminated by aliasing which can also be regarded as a ranging ambiguity problem. We have experimentally restored the 3D point clouds by aliasing synthesis with the assistance of intensity information. The ranging area can be elongated by at least ten times of the depth-of-focus with such data processing. With the above-all mentioned configuration and data processing, we have compromised the ranging ambiguity inherent in the amplitude-modulated continuous-wave laser scanner comprehensively. We expect that our results contribute to high-precision industrial inspection for Industry 4.0.
With the rise of Industry 4.0, smart factory is fast becoming a key concept in infrastructure. To realize the autonomous production system, it is necessary to ensure the parts are properly manufactured. 3D scanners are expected to play a vital role in quality assessment in smart factories. Especially, amplitude-modulated continuous-wave laser scanners benefit from high accuracy and high sensitivity which are suitable for industrial inspection. However, due to the limited dynamic range of receiver electronics, such laser scanners fail to obtain the data points in 3D measurement of highly reflective objects. This impairment deteriorates the performance of conversion of 3D point clouds to solid data for shape inspection, 3D modeling, reverse engineering etc. We coped with receiver saturation by adopting a high-speed polarizationindependent variable optical attenuator in our laser scanner. With such a lase scanner, we have succeeded in prevention of data loss due to receiver saturation.
High repetition rate with broadband spectrum optical pulses generated directly from an Yb – doped fiber oscillator were realized. The output spectrum has a 10 – dB spectral width of 106 nm over 1100nm. The average power of the pulses is higher than 100 mW and the repetition rate was 169.43 MHz. To the best of our knowledge, this is the first time the mode – locked Yb – doped fiber laser with the optical spectrum broader than 100 nm, pumped by low pump power has been reported.
Amplitude-modulated continuous-wave laser scanners can realize 3D measurement with high longitudinal resolution. With installation of focusing optics in the laser scanning system, the high lateral resolution can be realized, which is enhanced by the focused beam spot size <100 um. However, the depth-of-focus of the focusing optics is generally several cm. The 3D data of the defocused objects are contaminated by aliases distributed by integer times of the half cycle of the periodical modulation. Aliasing is an impairment inherent in the amplitude-modulated continuous-wave scheme. We experimentally recovered the defocused data drastically by synthesizing those aliases. The ranging area can be elongated by at least ten times with such data processing compared with the depth of focus. Our results will contribute to highprecision industrial inspection for Industry 4.0.
The performance of mid-infrared fiber lasers operating on the 3.5 μm transition in erbium has improved significantly since the first demonstration that dual wavelength pumping allowed efficient operation. In this contribution, we will discuss the progress of fiber lasers that operate on this transition with an emphasis on advances towards short pulse generation and wavelength agility. Mode-locked operation using saturable absorption is a robust means of achieving ultra-short pulse operation in the near infrared but achieving this in the mid-infrared has been elusive. We will also describe our characterization of the mid-infrared performance of graphene, a material which has been very successfully applied to mode-locked pulse generation in the near infrared.
We demonstrated distributed strain measurement using the slope-assisted Brillouin optical correlation-domain analysis (SA-BOCDA) with polarization maintaining fiber (PMF), which has no polarization scrambler. The 0.35- m-long strained section in 47-m-long PMF was clearly measured with 1-kHz sampling ratio and no averaging. Next, we proposed the new concept of the human interactive sound effector using SA-BOCDA with PMF, which means that the distributed measurement data was used as sound waveform directly. Then we demonstrated the sound wave controlling with SA-BOCDA with PMF. This result will be useful for not only the sensing application but also the musical instrument application based on nonlinear optical phenomenon.
Laser scanners have emerged as powerful instrument for high-precision 3D geometry measurement and high-resolution reverse engineering in combination with 3D printers etc. However, such attractive laser scanners have failed to address to fading phenomenon, which was a critical issue of laser scanning systems. Although extensive research has been carried out on the fading issue, no single study exists which effectively coped with such an impairment inherent in 3D geometry measurement using laser scanners. In this article, we propose a 3D laser scanner having a tunable high-speed polarization scrambling scheme and cope with the fading phenomenon. To our knowledge, this is the first report of versatile removal of the fading phenomenon inherent in laser scanning systems
We report an experimental study on the spectral dependence of depolarized guided acoustic-wave Brillouin scattering (GAWBS) in a silica single-mode fiber (SMF) on acoustic impedance of external materials. The GAWBS spectrum was measured when the acoustic impedance was changed from 1.51 to 2.35 kg/s⋅mm2. With increasing acoustic impedance, the linewidth increased; the dependence was almost linear in the range up to ∼2.0 kg/s⋅mm2 with an acoustic impedance dependence coefficient of 0.16 MHz/kg/s⋅mm2, which is directly applicable to acoustic impedance sensing. Meanwhile, with increasing acoustic impedance, the central frequency nonlinearly decreased, which probably originates from the change in the effective outer diameter of the silica SMF.
If an optical core network can be handled flexibly, it can be used not only as network infrastructure but also as a temporary broadband resource when customers have to transfer a large volume of data quickly, which will in turn lead to new WAN services. We propose "software-defined optical core networking", which achieves flexible optical network control, meaning it virtualizes optical transport network/wavelength-division multiplexing resources and controls them with resources from other layers, such as Ether/MPLS. We developed a testbed system and verified that users could request broadband resources easily, and our controller could quickly set up an optical channel data unit path for the request.
We review the optical properties of carbon nanotubes (CNTs) and graphene and describe how those properties have been
used for the implementation of various nonlinear fiber optic applications. Early studies on the optical properties of CNTs
in the late 90s revealed that these materials exhibit high third order susceptibility and a broadband saturable absorption
with a sub-picosecond response time. Recent discovery of similar nonlinear optical properties in graphene attracts much
attention in this field. Such ultrafast, highly nonlinear optical response means that they can be employed for noise
suppression and for the mode-locking of fiber lasers, and in addition, their high third order nonlinearity holds great
promise for the implementation of various other nonlinear fiber optic devices such as wavelength converters based on
four wave mixing. In this paper, we will discuss the various methods that have been considered thus far for the
integration of CNTs and graphene in optical systems and highlight the advantages and limitations of using the saturable
absorption of CNTs and graphene for the passive mode-locking of fiber lasers, and the current status of CNT and
graphene saturable absorbers in the state of art fiber laser technologies.
Generation of flat and broadband supercontinum is demonstrated in an all fiber system using the high-energy noise-like pulses from a stable figure-of-eight fiber laser and nonlinear fibers. This SC source is successfully applied to the spectral domain optical coherence tomography (SD-OCT). The axial resolution is significantly improved compared with the case of the superluminescent diode source. SD-OCT imaging is also demonstrated.
The broadband source with the arbitrary spectrum based on dispersion tuning technique was demonstrated. While a dispersion-tuned wavelength-swept laser is capable of fast and widely wavelength sweep, it can also be used as a broadband spectrally-flexible source by controlling the sweep waveform. It is the first demonstration of spectrallytunable source to the best of our knowledge. We used the dispersion-tuned laser as a broadband source for SD-OCT system by synchronizing sweep rate of a laser and exposure time of a CCD camera in SD-OCT system. We successfully obtained the images of an adhesive tape and a human finger.
Swept-Source Optical coherence tomography (SS-OCT) is a powerful tool for fast medical imaging. For the real-time 3D imaging, the wide tuning range over 100 nm and fast sweep rate over 100 kHz are typically required. We recently proposed a new wavelength-swept laser for SS-OCT. It is based on a principle called dispersion tuning. Since the cavity contains no mechanical components, such as tunable filters, we could achieve the very high sweep rate. In this review paper, we describe the principle of the dispersion-tuned swept lasers in detail and present our recent works on the application to the SS-OCT system.
We review the optical properties of carbon nanotubes (CNTs) and graphene and discuss how those properties can be
used in photonic applications. In particular, we will give an overview of the benefits of using their highly nonlinear
optical response in fiber lasers and other nonlinear fiber optic devices. Both graphene and CNTs exhibit high third order
susceptibility and a broadband saturable absorption with sub-picosecond response. We will discuss the advantages and
limitations of using the saturable absorption of carbon nanotubes and graphene for the passive mode-locking of fiber
lasers, introduce the different methods that we have developed to integrate these materials in the fiber system and
summarize the main contributions of these materials towards advancing fiber laser technology. In addition, these
materials also exhibit an extremely high third order susceptibility which is responsible for nonlinear processes such as
four wave mixing (FWM), Kerr focusing and third harmonic generation (THG) of great interest for optical switching and
wavelength conversion. The large absorption of CNTs and graphene however limits the dimensions of these devices and, thus, their applicability. We review our efforts towards enhancing and exploiting the nonlinearity of CNT and graphene fiber optics devices.
We have demonstrated a wavelength-swept fiber laser based on dispersion tuning method. In this method, the light in a
dispersive laser cavity is intensity modulated and actively mode-locked, and the lasing wavelength can be changed by
controlling the modulation frequency. As the dispersion-tuned laser does not include any tunable filters, the sweep rate
and range are not limited by mechanical moving parts. We have reported the wavelength-swept laser which has the
tuning range of over 100nm with the sweep rate of 200kHz, and we have applied the laser to the swept-source optical
coherence tomography (SS-OCT) system. Although we have successfully obtained the OCT image of the human finger
at 1kHz sweep rate, we could not obtain OCT images at higher sweep rate because of the performance degradation of the
laser. As this laser cavity included 100m long dispersion compensating fiber (DCF), the long laser cavity increased the
photon lifetime and resulted in the output power decrease and the linewidth broadening at higher sweep rate. In order to
solve these problems, we inserted a reflective semiconductor optical amplifier (RSOA) and a chirped fiber Bragg grating
(CFBG) into the laser cavity. Use of these devices made it possible to shorten the cavity length drastically and the laser
performance at high sweep rate is significantly improved. We could achieve that the sweep range of 60nm and the output
power of 8.4mW at 100kHz sweep. We applied the laser to swept-source OCT system and we successfully obtained
images of an adhesive tape at up to 250kHz sweep.
We demonstrate a wavelength-swept fiber laser based on the dispersion tuning method using a chirped fiber Bragg
grating (CFBG) and a reflective semiconductor optical amplifier (RSOA). The laser cavity length is reduced to as short
as 2m owing to the short and highly-dispersive CFBG, which is much shorter than the one using dispersion
compensating fiber (DCF). The short cavity enables higher sweep speed. We successfully obtain the OCT images of a
rolled adhesive tape at the sweep rate of 50kHz.
We proposed a wide and fast wavelength-swept fiber lasers based on the dispersion tuning for the optical coherence
tomography (OCT) applications. So far, we have achieved the sweep rate of ~200kHz at the sweep bandwidth of
~180nm. The sweep rate is only limited by the photon lifetime, which is proportional to the cavity length. Since we used
a dispersion compensating fiber (DCF) as the dispersive medium, the long cavity length (~100m) was the limit of the
sweep rate. In this paper, we demonstrate faster sweep rate up to ~500kHz by using a wideband chirped fiber Bragg
grating (CFBG).
A fiber Bragg grating (FBG) sensor using wide and fast mode-locked wavelength-swept laser is demonstrated. The key device is the optical source, wide and fast wavelength-tunable mode-locked fiber laser based on dispersion tuning. This laser has wide tuning range (over 100nm) and a fast tunable speed (up to 20 kHz). Hence, it is suited for the dynamic FBG sensor system. Our sensor system enables dynamic measurement of strain in real-time at a high measurement rate. In this system,the signals of the sensor are obtained by analyzing temporal waveforms of refleceted light from the FBG without observing the spectrum. A measurement speed as high as 1 kHz is achieved when the wavelength-tunable source is swept at 1 kHz rate. The stability (indicated by the standard deviations of the results) is 30-60 pm in optical spectrum.
We recently proposed and demonstrated a saturable absorber (SA) incorporating carbon nanotube (CNT). CNT-based
SA offers several key advantages such as: ultra-fast recovery time, polarization insensitivity, high optical damage
threshold, mechanical and environmental robustness, chemical stability, and the ability to operate at wide range of
wavelength bands. Using the CNT-based SA, we have realized femtosecond fiber pulsed lasers at various wavelengths,
as well as the very short-cavity fiber laser having high repetition rate. Besides the saturable absorption, CNT has been
shown to have high third-order nonlinearity, which is also attractive for realization of compact and integrated functional
photonic devices, such as all-optical switches and wavelength converters. In this paper, we first present photonic
properties of CNTs, and review our studies on CNT-based mode-locked fiber lasers. We also refer to fabrication
methods of CNT-based photonic devices. We show our recent research progresses on novel photonic devices using
evanescent coupling between optical field and CNT.
Passively mode-locked fiber lasers are the best pulsed sources available today due to their simplicity and their ability to
generate transform-limited pulses in the picosecond regimes. A drawback of the conventional passively mode-locked
fiber lasers is that the pulse repetition rate is relatively low, at best a few tens of MHz, because of long cavity length. In
order to raise repetition rate up to a few GHz, the cavity length has to be shortened below a few centimeters. Fiber lasers
with such a short cavity require a high gain fiber and a small saturable absorber with low loss. Recently, the authors
have proposed and demonstrated a small and low-loss saturable absorber device incorporating carbon nanotubes (CNT).
Using CNT, we have realized very stable 2cm-long, 5GHz mode-locked Er:Yb-codoped silica-fiber laser, but the output
power was limited to ~0.2mW due to insufficient gain in the Er:Yb-codoped silica-fiber. Here we used heavily Er:Yb-codoped
phosphate fiber to form 1cm-long cavity with fiber mirrors, and succeeded in generating stable pulse trains
with output power as high as 30mW and repetition rate as high as 10GHz at 1535nm.
Carbon nanotubes (CNTs) have been intensively studied for optical applications because of their useful characteristics.
However, handling of the CNTs is one of the largest problems for device applications. Several methods have been
reported to fabricate optical devices, such as spray method, direct synthesis method, and polymer embedding method.
These methods require complicated process and dissipate excessive amount of CNTs. Therefore, an easy and cost
effective handling technique of CNTs is required. In this paper, we propose and demonstrate a novel technique to deposit
CNTs onto only the core region of end facets of optical fibers. We successfully realized area selective deposition using
optical tweezers. This technique requires a very simple setup and consumes only a small amount of CNTs. We confirmed
presence of CNTs at the selected region by microscopic Raman spectroscopy. As an optical device application, we
inserted the CNT deposited fiber into the fiber ring laser cavity as a saturable absorber, and realized passive modelocking.
This technique will allow us to realize low-cost CNT-based photonic devices.
We demonstrate a wide and fast wavelength-tunable mode-locked fiber laser based on tuning the mode-locking
frequency. The laser is in a sigma-laser configuration and a wideband semiconductor optical amplifier (SOA) at 1.3 μm
wavelength region is used as a gain medium. Mode-locking is achieved by direct modulation of the injection current to
the SOA, and a dispersion compensation fiber (DCF) is used to provide desired intracavity dispersion. By tuning the
modulation frequency, a wide tuning range over 100 nm is achieved. Lasing wavelength is measured to be in linearly
proportion to the RF frequency applied to the SOA. The sweep rate over the entire wavelength range (100 nm) can be
raised to be as high as 100 kHz.
In this paper, a novel technique for multiplexed fiber Bragg grating (FBG) sensors is proposed. Applying the synthesis of optical coherence function, we can select the signal, which has a specific time delay with the reference light wave, and therefore FBG array with the same reflection wavelength can be used.
We propose and demonstrate a novel quasi-distributed fiber laser stress sensor. A stress position is located by the FSR shift induced by mode coupling in an intracavity PMF. Resolution of 54cm in the 150m-range could be realized with the mode-locking technique.
Brillouin Fiber Optic Gyro (B-FOG) can detect angular velocity from the beat frequency between the two lasing Stokes lights that circulate in opposite directions in the fiber ring resonator. This means the B-FOG has inherently the beat output format suitable for navigation applications. However, B-FOG has a serious problem called 'lock-in phenomenon'. In lower rotation rate, frequency pulling effect occurs between the two Stokes lights due to backscattering in the resonant, and the beat signal is getting distorted and finally locked to zero. In this report, at first, we realize B-FOG and observe its output characteristics, including optical Kerr-effect induced bias and the lock-in phenomenon. Then, the details of the lock-in phenomenon are evaluated theoretically. Additionally, we propose a novel method to avoid the lock-in phenomenon by modulating the gyro output through the optical Kerr-effect in the fiber resonator. Finally, we verify this method experimentally.
Er doped fiber ring laser without mode instability due to polarization fluctuation is proposed and experimentally investigated. The resonator composed of a
twisted Er doped fiber and a coiled fiber ?/2 element can fix the frequency-separation between two polarization-modes.
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