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The effect of treating a ZrF 4 -based fluoride-glass surface with hydrogen fluoride gas is investigated using secondary-ion mass spectroscopy. The analysis reveals that this treatment is effective in reducing the oxygen concentration at the surface. A strong fiber with a mean tensile strength of 550 MPa and maximum strength of 600 MPa is successfully fabricated by using this technique.

This paper describes the amplification characteristics of gain-flattened Er/sup 3+/-doped fiber amplifiers (EDFAs) by using 0.98-/spl mu/m and 1.48-/spl mu/m band pumping for a 1.58-/spl mu/m band WDM signal. Silica-based Er/sup 3+/-doped fiber (S-EDF) and fluoride-based Er/sup 3+/-doped fiber (F-EDF) have gain-flattened wavelength ranges from 1570 to 1600 nm and from 1565 to 1600 nm, respectively, and exhibit uniform gain characteristics with gain excursions of 0.7 and 1.0 dB, and the figure of merit of the gain flatness (gain excursion/average signal gain) of 3 and 4.3%, respectively, for an eight-channel signal in the 1.58-/spl mu/m band. We show that 1.48-/spl mu/m band pumping has a better quantum conversion efficiency and gain coefficient, and that 0.98-/spl mu/m band pumping is effective for improving the noise characteristics. We also show that the EDFAs consisting of two cascaded amplification units pumped in the 0.98-/spl mu/m and 1.48-/spl mu/m bands are effective in constructing low-noise and high-gain 1.58-/spl mu/m band amplifiers.

This paper describes the development of an efficient praseodymium-doped fluoride fiber amplifier (PDFA) using newly developed PbF/sub 2/-InF/sub 3/-based fluoride single-mode fiber. First, the spectroscopic properties of Pr/sup 3+/ in PbF/sub 2/-InF/sub 3/-based and ZrF/sub 4/-based fluoride glass are compared and then the fabrication of high-NA PbF/sub 2/-InF/sub 3/-based fluoride single-mode fiber is presented. The gain coefficients obtained with the developed fiber are 0.36 dB/mW with a forward pumping scheme and 0.4 dB/mW with a bidirectional pumping scheme. We use this newly developed fiber to construct two types of an efficient PDFA module. We realized a 19-in rack size PDFA module with a master oscillator/power amplifier laser diode (MOPA-LD) and a plug-in type PDFA module with a wavelength-stabilized 1.017-/spl mu/m LD and describe their amplification characteristics.

The PbF2–GaF3–InF3–ZnF2–YF3–LaF3 glass system is developed for potential use as an efficient Pr3+-doped 1.3 μm fiber amplifier (PDFA) host. It is shown that 38PbF2–25GaF3–13InF3–16ZnF2–4YF3–4LaF3 (mol%) glass has a glass transition temperature of 252±1°C, a crystallization temperature of 350±1°C, a refractive index (nD) of 1.597±0.001 and an infrared transmission window up to 8 μm. We confirm that its thermal stability against crystallization is similar to that of stable ZrF4-based fluoride glasses. An optical fiber with a minimum transmission loss of 43 dB/km at 3.36 μm is successfully fabricated using this glass.

A modified jacketing method, which uses a preform produced by suction casting and a tapered jacketing tube for fiber drawing, was developed to fabricate low-loss, high-Δn fluoride single-mode fibers for optical amplifiers. A fabricated 500 m long ZrF4-based fiber had a constant core diameter and an almost constant mean strength of ~435 MPa over its whole length. Low transmission losses below 20 dB/km were achieved in rare earth-doped ZrF4-based single-mode fibers with a Δn of 3.7%. An InF3-based single-mode fiber with a Δn of 3.6%, a loss of 114 dB/km at 1.23 μm and a signal gain coefficient of 0.18 dB/mW at 1.30 μm was fabricated using this jacketing method.

Recent progress in fiber amplifier technology is reviewed. Broad band and high power are currently the key goals in the field of fiber amplifiers in conjunction with amplified system development. Broad-band EDFAs using fluoride-based hosts and high-power EDFAs with powerful pump lasers are described in detail. Recent progress and other requirements for optical amplifiers i.e. high gain, low noise, and high reliability, are all reviewed. The limitations in relation to achievable gain are described as well as the highest gain ever achieved. A method for its reduction are described. In relation to reliability, this paper mainly focuses on fluoride fiber. Recent progress on the main characteristics of 1.3 µm PDFA is reviewed and transmission experiment are described. Several attempts to construct fiber amplifiers operating at 0.8 µm, 1.4 µm, and 1.65 µm are also reviewed. In the field of amplified systems, WDM and remote pumping TDM techniques are a high priority. In particular, various WDM experiments such as long-distance WDM transmission, WDM soliton transmission, and WDM networks are all described in detail.

The feasibility of using phosphate glasses as overcladding materials for fluoride glass fibers was investigated by measuring their thermal properties, corrosion rate and mechanical strength. In this experiment, Al 2 O 3 -doped phosphate glasses were developed as a new oxide glass material. Both the drawing temperature and the thermal expansion coefficient of phosphate glasses containing a small amount of Al 2 O 3 were found to be the same as those of fluoride glasses. In addition, it was confirmed that these phosphate glasses have good mechanical strength and chemical durability.

Numerical analysis reveals that a bi-directional pump scheme suppresses the population reduction of the /sup 1/G/sub 4/ level due to pump ESA and cooperative upconversion. It is confirmed experimentally that the gain coefficient increases from 0.21 to 0.24 dB/mW, which is the maximum single pass gain coefficient ever reported, with the bi-directional pump scheme. It is shown that this pump scheme is effective in extracting the potential of Pr/sup 3+/-doped fluoride fibers. >

A praseodymium (Pr)-doped fluoride fiber amplifier (PDFA) module that is pumped by strained quantum-well InGaAs laser diodes (LDs) is described. The amplifier module, consisting of a four LD pump configuration and a high NA Pr-doped fluoride fiber with low scattering loss, exhibits a maximum signal gain of 28.3 dB and a saturation output power of 6 dBm at a signal wavelength of 1.30 mu m. It is shown to be the most promising module for the 1.3- mu m-band optical amplifier. >

Coupling characteristics between two curved waveguides are studied using a two-core fiber. A tunable monochromatic light source is used to measure the wavelength dependence of the coupling efficiency. Measured values of the coupling efficiency agree well with theory. It is found that (1) the coupling efficiency can be changed from 0 to 1 by changing the radius of curvature, for example, from 100 mm to infinity in the measured coupler; and (2) when phase shift due to curvature sbeta/2R exceeds the coupling coefficient K, where s is the center-to-center core separation, beta the propagation constant, and R the radius of curvature, coupling between the curved cores does not occur, and there is only the phase shift between the cores.

The lifetime of the 1G4 level and the 1G4 –3H5 emission of Pr3+ are measured under changing temperatures. The temperature dependence of gain characteristics of Pr3+-doped fiber amplifiers is analyzed theoretically for the first time to our knowledge on the basis of the lifetime and emission cross section. Signal output power degradation resulting from an increase in temperature is clarified, and the pump power margin to compensate for this signal output power degradation is obtained.

Frequency stabilization of an 1.5 μm distributed feedback laser diode (DFB-LB) is demonstrated using a vibrational-rotational absorption of acetylene 12C2H2 and 13C2H2 molecules (VRAM). Frequency stability within 2 MHz peak-to-peak fluctuation can be achieved in the wavelength region of 1.51–1.55 μm. Frequency-stabilized distributed feedback (DFB) laser compact modules have been constructed. When the fiber-pigtail-type acoustic optical modulator was used for small laser frequency scanning, frequency stability within 3 MHz peak-to-peak fluctuation was achieved without any broadening of laser spectral linewidth. In addition, temperature and pressure dependencies of the acetylene absorption lines are characterized.

An optical fiber with sharp optical absorption peaks in the wavelength region from 1.515 to 1.535 mu m is discussed. These absorption peaks are provided by the vibrational-rotational absorption of C/sub 2/H/sub 2/ molecules (VRAM) placed inside the small vacant hole of the core center. Each peak has a FWHM bandwidth of 9.8 GHz and an absorption of between 1 and 5 dB for the 300-m-long fiber. >

We have clarified the strength and estimated lifetime of fluoride fibers for optical amplifier use. We fabricated a UV-curable epoxy-acrylate coated fluoride fiber with a strength of 542 MPa. We estimated the lifetime of the fluoride fiber to be longer than 25 years at 80/spl deg/C and 50% RH.

Optical amplification at 1.083 μm with neodymium-doped chalcogenide fiber is observed. The thermal and spectroscopic properties of a new chalcogenide glass system are measured in view of its potential use as a rare-earth host. A maximum internal gain of 6.8 dB is achieved for a pump power of 180 mW.

We constructed the first Pr/sup 3+/-doped fluoride fiber amplifier (PDFA) module pumped by a fiber-coupled master oscillator/power amplifier laser diode (MOPA-LD) operating at 1.017 /spl mu/m. The maximum signal gain and noise figure were 30.5 and 5.5 dB, respectively. An output power of 18 dBm was achieved at an input signal power of 0 dBm. Furthermore, we showed that the use of MOPA-LD pumping, rather than conventional Nd-YLF laser pumping, makes it possible to halve the length of the Pr/sup 3+/-doped fluoride required in the amplifier module and also allows a broader spectral bandwidth to be achieved.

The highest reported single-pass gain coefficient of 0.36 dB/mW has been achieved using a newly developed Pr/sup 3+/-doped high-NA PbF/sub 2//InF/sub 3/-based fluoride fiber, with a /spl Delta/n of 6.6%, a core diameter of 1.2 /spl mu/m and a transmission loss of 250 dB/km at 1.2 /spl mu/m. This fiber was used to construct an efficient PDFA module with a MOPA-LD. A small-signal net gain of 22.5 dB was achieved at 1.30 /spl mu/m with a pump power of 23m mW.

We successfully developed a fluoride-based Er/sup 3+/-doped fiber amplifier (F-EDFA). An average signal gain of 26 dB was achieved for 8 channel wavelength division multiplexed (WDM) signals in the 1532-1560 nm wavelength region with a gain excursion of less than 1.5 dB at an input signal power of -20 dBm per channel. Furthermore, we studied the amplification characteristics of the F-EDFA for WDM signals. The following experimental results were obtained. (1) For an 8-channel WDM signal in the 1532 to 1560 nm wavelength region, the gain excursion between channels can be suppressed to within 1.5 dB. However, the wavelength region allowing a gain excursion of 1.5 dB, is between 1536-1560 nm for the silica-based Er/sup 3+/-doped fiber amplifier. (2) F-EDFAs have a flat gain region between 1534-1542 nm. The gain excursion of this region is less than 0.2 dB for WDM signals.

We propose a novel low noise and gain-flattened Er/sup 3+/-doped fiber amplifier (EDFA) with a cascade configuration for wavelength division multiplexing (WDM) signals. In this configuration, a 1480-nm pumped fluoride-based EDFA is joined to a 980-nm pumped silica-based EDFA through an optical isolator. By adjusting the silica-based Er/sup 3+/-doped fiber length in the silica-based EDFA, we realized an excellent flat gain EDFA with a gain excursion of less than 0.9 dB and noise figure of 5.7/spl plusmn/0.2 dB, and a low noise EDFA with a noise figure of 5/spl plusmn/0.2 dB and a gain excursion of less than 1.4 dB, for 8 channel WDM signal in the 1532-1560-nm wavelength region.

Signal gain and noise figure in a bi-directional pumping configuration are investigated in the unsaturated and saturated region. Balanced and asymmetric bi-directional pumping configurations are effective in the unsaturated and saturated region, respectively. Especially, an asymmetric bi-directional pumping configuration with larger backward pump power than forward pump power increases the pump to signal conversion efficiency in the saturated region. An asymmetric pumping configuration is effective in constructing booster amplifiers.

We demonstrate the first high gain rare-earth-doped fiber amplifier operating at 1.65 /spl mu/m. It consists of ZBLYAN fiber with a Tm/sup 3+/-doped core and Tb/sup 3+/-doped cladding, pumped by 1.22 /spl mu/m laser diodes. It is possible to achieve efficient amplification with Tm/sup 3+/ ions if their amplified spontaneous emission (ASE) in the 1.75 to 2.0 /spl mu/m wavelength region is suppressed by doping Tb/sup 3+/ ions in the cladding. A two-stage-type fiber amplifier is constructed and a signal gain of 35 dB is achieved for a pump power of 140 mW. A gain over 25 dB is realized in the 1.65 /spl mu/m to 1.67 /spl mu/m wavelength region.

The crystallization properties of ZrF4‐based fluoride glasses containing PbF2 are clarified and single‐mode fibers for an optical amplifier operating at 1.3 μm are fabricated. The crystallization tendency increases as the PbF2 content of the glasses increases and the optimum PbF2 content ranges from 11 to 12.5 mol % in terms of glass stability and refractive indices. The 4%‐Δn single‐mode fiber with a transmission loss of 44 dB/km at 1.2 μm was prepared as the amplifier host fiber. ZrF4‐based glass with a PbF2 content of 11 mol % was used as a core glass and HfF4‐based glass was used as a cladding glass.

The gain characteristics of a 1.4-/spl mu/m-band thulium-holmium-doped ZBLYAN fiber amplifier are described. Signal gain is obtained over the whole 1.4-/spl mu/m band for a pump power level of 73.5 mW. A maximum signal gain of 18 dB is achieved at a signal wavelength of 6.46 /spl mu/m for a pump power of 150 mW at 0.79 /spl mu/m. The noise figure is 5.6 dB in the signal wavelength region from 1.45-1.50 /spl mu/m. From a comparison of the gain characteristics of thulium-holmium-doped ZBLYAN fibers and thulium-doped ZBLYAN fibers, it is proved that holmium ions play an effective role in increasing the gain and widening the gain spectrum. >

We have successfully developed a low-noise and high-power Pr/sup 3+/-doped fluoride amplifier (PDFA) module with a cascade configuration. The maximum signal gain and noise figure were 40.6 dB and 5 dB, respectively. An output power of 20.1 dBm was achieved at an input signal power of 0 dBm. We confirm that this PDFA module performs well from experimental results on its use in a 40 channel AM-VSB video signal transmission and a 10 Gbit/s digital transmission. >

In this paper we will explain the preparation of an epitaxially grown ytterbium‐ and neodymium‐co‐doped yttrium aluminum garnet [(Yb,Nd):YAG]–buried channel waveguide laser on a YAG substrate. Waveguides, whose cores are formed with Ar ion beam etching, demonstrate single‐mode operation. The (Yb,Nd):YAG waveguide laser operates at 1.03 μm when pumped at 0.81 μm, as a result of a nonradiative energy transfer from Nd3+ ions to Yb3+ ions. The incident threshold and slope efficiency of the 5 mm long waveguide laser are 235 mW and 13%, respectively. Output power of 50 mW is obtained for an incident power of 620 mW at room temperature.

Pr3+‐doped As‐S chalcogenide fiber is fabricated using a double‐crucible method. The thermal and spectroscopic properties of Pr3+‐doped As‐S glasses are clarified. Pr3+‐doped As‐S fiber is shown to be a promising candidate that will lead to efficient Pr3+‐doped fiber amplifiers.

A highly efficient Pr/sup 3+/-doped fluoride fiber amplifier configuration with an optical circulator, in which the input signal light is amplified both forward and backward through a Pr/sup 3+/-doped fluoride fiber, is investigated with a view to decreasing the drive current and improving the reliability of pump laser diodes (LD's). With this double-path configuration, a 25-dB signal gain is achieved at an LD drive current of 110 mA. This LD drive current is about half that needed for a conventional single-path configuration. It is also found that this double-path configuration provides a double-gain coefficient, a slightly low saturation power, and a slightly narrow spectral gain width. >

Using unclad Nd:YAG crystal fibers of 10-40 mu m diameter and 1 cm long, and a laser diode pumping power of 83 mW at a wavelength of 0.81 mu m, gains of up to 6.5 dB at 1.32 mu m have been observed. No significant dependence of gain on signal polarization or output signal level between -43 and -5.4 dBm was observed for a 16 mu m diameter fiber. For smaller diameter fibers and a reduction of the competition from amplified spontaneous emission (ASE) at 1.064 mu m, much higher gains are expected. Extrapolation of experimental and theoretical results predicts gains in excess of approximately 30 dB for fiber diameters of 5 mu m. >

We demonstrate that the self‐organized χ(2) grating in a high germania content optical fiber can be erased by prolonged exposure to 1.064 μm radiation only. The decay rate, which is proportional to the fourth power of the 1064 nm intensity, is attributed to two‐photon absorption of the second‐harmonic light generated in the fiber.

This paper describes the optical amplification characteristics and reliability of praseodymium-doped and erbium-doped fluoride optical fiber amplifiers, with a view to their application in practical optical systems.

This paper reviews recently developed ultra-broadband EDFAs which include an amplifier composed of a 1.55 μm-band and a 1.58 μm-band gain-flattened EDFA in a parallel configuration and an Er3+-doped tellurite fiber amplifier. Furthermore, it dcscribes improvements in the 1.55 μm-band gain-flattened fluoride-based EDFA.

The fiber amplifier is now established as a key component for practical optical transmission systems because of its excellent amplification characteristics.

The construction of economical broadband optical-communication networks requires practical optical amplifiers with high output power and low noise at the zero-dispersion wavelength of the transmission fiber. Praseodymium-doped fluoride-fiber amplifiers (PDFAs) could be significant in improving the 1.3-µm optical transmission system, which is the most widely installed terrestrial system.1,2 In this paper we report on a high-power, low-noise PDFA with a cascade configuration and give its transmission characteristics.

In the past year, many attempts have been made to improve the characteristics of praseodymium-doped fiber amplifiers (PDFA) and to apply them to practical systems. The goals of those experiments were to obtain a longer lifetime with lower phonon energy material1,2 to achieve a higher gain PDFA module with a lower loss host fiber,3 to realize higher output power with higher pump power, and to clarify the amplification characteristics for transmission applications such as CATV and long- haul transmission.5,6,7 On the basis of our investigations, we summarize recent progress in the improvement of the PDFA characteristics and in the application of the PDFA to practical systems, and also discuss the future prospects for PDFA.

The advent of the erbium-doped fiber amplifier has led to the construction of many attractive transmission systems in the 1.5 µm region such as ultra-long distance systems(1)(2), large capacity CATV systems(3)(4), and optical soliton transmission systems(5). On the other hand, the 1.3 µm transmission system, where most terrestrial fiber transmission systems installed around the world operate, makes great demands on the development of amplifiers operating at 1.3 µm region. To date, however, there have been no practical amplifiers developed yet.

The Pr3+-doped fiber amplifier (PDFA) is a candidate for use as an optical amplifier in 1.3 µm transmission systems(1). A recent paper reporting that a signal gain of 28.3 dB was obtained by laser diode pumping strongly suggests that the PDFA is a device with the potential to be applied in 1.3 pm optical systems(2).

The early explorers of laser technology expected that “the amount of information that can be carried by a communication channel is proportional to its frequency, and in principle the visible region of the spectrum between the wavelengths of 4,000 and 7,000 angstrom units could accommodate 80 million television channels.

When ultrashort pulses of below a few hundred femtoseconds are transmitted through an optical fibers, they are greatly affected by the dispersion and the length of that fibers.

Pr-doped fluoride fiber(PDF) is the most promising fiber-type gain medium for the 1.3 µm fiber amplifiers(1)-(3). However, to date, Pr-doped fluoride fiber amplifier (PDFA) module pumped by laser diodes (LD's) has not been developed yet. The difficulties in constructing PDFA compact modules are as follows: To achieve a high signal gain, it is necessary to develop a pump power of more than 100 mW and a high numerical aperture (NA) fiber structure, because a PDF has the low quantum efficiency of the 1.3 µm-band stimulated transition (1G4-3H5) as yet. Furthermore, it is difficult to achieve a low-loss, stable splice between high NA PDF and silica fiber.

Pr3+-doped fiber amplifiers show promise for use as optical amplifiers operating at 1.3 mm. They have generated immense interest in the field of telecommunications because most current optical fiber systems operate at wavelengths around 1.3 mm, which is the second telecommunication window.

Frequency stabilization of laser diodes is of vital importance to the exploitation of frequency division multiplexing coherent transmission systems. In particular, frequency stabilization in the 1.5- μm region, ideally at 1.55 μm of importance for present optical transmission systems, is now required. For frequency stabilization of 1.5-μm DFB lasers, several schemes have been investigated which involve use of an ammonia absorption line1 at 1.519 μm, a krypton optogalvanic effect2 at 1.533 μm, or second harmonic generation of 1.56-μm laser diode radiation and an atomic rubidium line.3 Another method using acetylene (C2H2) absorption lines as a frequency reference has been investigated in the region from 1.52 to 1.54 μm4,5 However, since the absorption of C2H2 molecules in the wavelength region longer than 1.54 μm is rather weak,4,5 it is difficult to use this molecule for practical frequency stabilization at these longer wavelengths. In fact, to date, no frequency references suitable for stabilization in the regions of 1.54 and 1.55 μm have been proposed, and no demonstration of frequency stabilization has been reported.

The authors have constructed a Pr/sup 3+/-doped fluoride amplifier (PDFA) module pumped by an Nd-YLF laser. The maximum signal gain and noise figure were 20 dB and 5 dB respectively. An output power of 19.2 dBm was achieved at an input signal power of 11.0 dBm. It was confirmed that this PDFA module has low-noise characteristics from experimental results on its use in a subcarrier multiplexed multichannel AM-VSB video signal transmission.< >