Fiber-in-the-loop Research Articles

An EDFA-Gain Equalizer Based On a Sagnac Loop With an Unpumped Erbium-Doped Fiber

We have demonstrated an erbium doped fiber amplifier (EDFA) gain equalizer for flattening the amplified spontaneous emission gain spectrum using a Sagnac loop incorporated with a single-section unpumped erbium-doped fiber. The gain spectra output from the transmission port and the reflection port of the Sagnac loop are studied in detail. The absorption effect of unpumped erbium-doped fiber and the resonance mode caused by the birefringence beat length in the loop are combined to make the gain spectrum be partially or completely flattened by adjusting the polarization controllers properly in the loop. On the one hand, the part of the spectrum is flatted within ±0.15 dB and ±0.145 dB in the wavelength range of 16 nm and 14 nm at the reflection port and transmission port respectively. On the other hand, the total spectrum is flatted within ±1.225 dB over a bandwidth of 36.5 nm in the whole C-band at the transmission port, while the two-peak with 29.6 nm interval occurred at the reflection port because of the 0.15-1.55 m birefringence beat length of single-mode erbium-doped fiber in the loop. The results are consistent with theoretical simulations. This flat technique has the potential to be used in communication technology, especially in wavelength division multiplexing to adjust multichannel gain simultaneously and multi-wavelength narrow-linewidth fiber lasers to achieve a wide range of stable light output.

Hybrid Frequency-Tunable Parity-Time Symmetric Optoelectronic Oscillator

An optoelectronic oscillator (OEO) is a microwave photonic system consisting of an amplified optoelectronic feedback loop to generate a microwave signal. To reduce the phase noise, the feedback loop must be long to ensure a high Q factor, which is usually implemented by incorporating a long optical fiber in the loop. The key limitation of using a long fiber is that a large number of closely spaced longitudinal modes exist in the feedback loop, which makes single-frequency oscillation difficult to achieve. In this article, we propose to incorporate a photonic integrated microdisk resonator (MDR) in an OEO to implement parity-time (PT) symmetry as well as frequency tuning. By employing the reciprocity of light propagation in the MDR, two mutually coupled optoelectronic loops having an identical geometry with one having a gain coefficient and the other a loss coefficient, identical in magnitude, to form a PT-symmetric OEO that support single-frequency oscillation, are implemented. The tuning of the frequency is realized by thermally tuning the MDR. Experimental results show a microwave signal with a frequency that is tunable from 2 to 12 GHz is realized. The phase noise of the generated microwave signal at a frequency of 11.5 GHz is −117.3 dBc/Hz at a 10-kHz frequency offset. The use of a photonic integrated microdisk resonator makes the entire PT symmetric system have high potential for full photonic integration.

Self-Q-Switch and CW Operation of a Tunable Dual-Wavelength Er/Yb Double-Clad Fiber Laser

In this paper, a double-clad Er/Yb fiber laser with self-Q-switched and continuous wave operation depending on the pump power range is experimentally demonstrated. The linear cavity is formed on one side by a pair of cascaded tunable fiber Bragg gratings used for the selection and tuning of the generated laser lines. On the opposite side, a fiber optical loop mirror with high birefringence fiber in the loop is used to adjust the intra-cavity losses to obtain dual-wavelength emission by temperature changes on the fiber loop. Continuous wave dual-wavelength laser operation is obtained for tunable separation of the generated laser lines in a range from 1 to 7 nm, maximum output power of 3.6Wwith a pump power of 10Wand laser wavelengths linewidth of ~0.2 nm. Self-Q-switched laser pulses are obtained with low pump power in a range from 322 to 890 mW. Q-switched pulses with minimum pulse duration of ~1.5 _s and maximum pulse energy of ~3.5 _J are obtained.

All-fiber radio frequency-modulated pulsed laser based on frequency-shift feedback loop

Lidar-radar by using an radio frequency modulated (RF-modulated) laser transmitter is a powerful technique for applications involving remote sensing. The method is based on the use of an optically carried RF signal in order to acquire the merits of both the directivity of the optical beam (lidar) and the accuracy of RF signal processing (radar). Compared with single-frequency coherent lidars, lidar-radars are less sensitive to atmospheric turbulence and the speckle noise induced by target roughness. For long range detection, pulsed operation is usually required because of the high peak power. In order to meet the requirement for long range detection, an RF-modulated pulse train based on an all-fiber frequency shifted feedback loop is proposed in this paper. A continuous-wave single-frequency fiber laser (seed laser) is coupled into a fiber link and an acousto-optic chopper is used as a frequency shifter and beam chopper. A Yb3+-doped fiber amplifier is used to compensate for the loss of the signal in the fiber loop. The pulse duration is determined by the open time of acousto-optic chopper, which is fixed at 110 ns. A square wave generated by an arbitrary waveform generator is used as a trigger signal of the acousto-optic chopper. The RF within the pulse results from the interference of frequency shifed pulse with the seed laser. By inserting a 10 km fiber in the loop and accurately controlling the trigger cycle of the acousto-optic chopper equal to the roundtrip time of the loop, the pulse train generated by acousto-optic chopper can circulate in the loop, leading to the generation of RF-modulated pulse with about 20 kHz repetition rate and 110 ns width. The gain provided by fiber amplifier in the loop partially compensates for the loss. By adjusting the gain of fiber amplifier, the modulation depth of RF within the pulse can be continuously adjusted and the maximum modulation depth is 0.67. We also present an time-delayed scalar interference model which includes the loop length, trigger cycle, frequency-shift, and the gain. According to the theoretical model, the RF-modulated pulse affected by trigger cycle and fiber amplifier is numerically simulated. The experimental results accord well with theoritical predictions. The RF-modulated pulse has the advantage of high pulse-to-pulse coherence, which provides potential applications in lidar-radar detection. Besides, with an additional frequency doubling stage one can obtain a source for underwater detections and communications. Extension of the scheme to the 1.5 μm telecommunication window is straightforwardfor various radio-over-fiber applications.

Versatile patterns of multiple rectangular noise-like pulses in a fiber laser.

We report on the generation of versatile patterns of multiple rectangular noise-like pulses (NLPs) in a fiber laser mode-locked by nonlinear amplifying loop mirror (NALM). Benefiting from the strengthened nonlinear effect of a segment of highly nonlinear fiber (HNLF) in the loop, multiple rectangular NLPs with various patterns are formed depending on the cavity parameter settings. In particular, the multiple rectangular NLPs could possess unequal packet durations, which is different from the conventional multi-soliton patterns. The experimental results contribute to further understanding the characteristics of the rectangular NLP and the dynamics of multi-pulse patterns.

Self-Q-switched Er–Yb double clad fiber laser with dual wavelength or tunable single wavelength operation by a Sagnac interferometer

We report a self-Q-switched Erbium-Ytterbium-doped double cladding fiber ring laser with dual wavelength or tunable single wavelength operations. A Sagnac interferometer with a high birefringence fiber in the loop was used for the wavelength tuning of the single line operation and cavity loss adjustment for dual wavelength laser operation. Single wavelength laser operation for a pump power of 421 mW tunable in a range of 1561.4 nm to 1569.8 nm and dual wavelength laser operation at 1561.1 nm and 1571.4 nm with equal output powers are presented.

Actively Q-switched dual-wavelength laser with double-cladding Er/Yb-doped fiber using a Hi-Bi Sagnac interferometer

An actively Q-switched double-wavelength Er/Yb fiber laser is experimentally demonstrated. The linear cavity is formed by a pair of fiber Bragg gratings on one side and a Sagnac interferometer (SI) with high birefringence fiber in the loop on the opposite side. A 3 m of double-cladding Er/Yb-doped fiber used as a gain medium is pumped by a 978 nm laser diode. The SI is used to adjust the internal cavity losses for simultaneous dual-wavelength laser generation. The adjustment is performed by temperature variations of the high birefringence fiber in the SI loop. The maximum average output power for the Q-switched laser operation in dual-wavelength mode was around 68 mW with a repetition rate of 40 kHz for 2 W of pump power. The minimum pulse duration was around 280 ns. The maximum pulse energy was 1.75 µJ.

All-Fiber Low-Pedestal Spectral Compression in a Novel Architecture Based on an SMF Cascading an HNLF-NOLM

A novel all-fiber low-pedestal spectral compression scheme is proposed and demonstrated. The scheme is based on an anomalously dispersive single-mode fiber (SMF) cascading a nonlinear optical loop mirror with a highly nonlinear fiber (HNLF) in the loop. Both numerical and experimental results show that the spectral pedestal after spectral compression in the HNLF can be efficiently suppressed by the nonlinear optical loop mirror through the chirp-related intensity filtering effect. The measured spectral pedestal energy ratio is 9.59% using the proposed scheme, which is nearly a quarter of that using the conventional alternative based on an anomalously dispersive SMF followed by a feedthrough HNLF.

All-fiber passive mode-locked laser to generate ps pulses based in a symmetrical NOLM

We experimentally investigate an all-fiber passively mode-locked laser generating ps pulses. The experimental setup is a figure eight fiber laser configuration, including a power-symmetric Nonlinear Optical Loop Mirror (NOLM) with highly twisted low-birefringence fiber in the loop. NOLM switching is achieved by polarization asymmetry between the counter-propagating beams in the loop. We used a Quarter-Wave Retarder in the loop to break the polarization symmetry. Using a polarizer beam-splitter cube as the NOLM output we got the best quality output pulses from the laser. At this output, we are monitoring the output pulse polarization component which is parallel to the input NOLM component. We achieved stable generation of ∼25 ps pulses at the repetition frequency of 0.78 MHz with milliwatts average output power. The mode-locked laser ran in stable operation for hours.

Experimental investigation of a passively mode-locked fiber laser based on a symmetrical NOLM with a highly twisted low-birefringence fiber

We experimentally investigate the passive mode-locking operation of a figure eight-fiber laser based on a symmetrical nonlinear optical loop mirror (NOLM) with a highly twisted low-birefringence fiber in the loop. NOLM switching is achieved by the polarization asymmetry between the counterpropagating beams in the loop. The most efficient switching is obtained when we have linear polarization for one of the beams and the circular polarization for the other. We used a quarter-wave retarder (QWR) in the NOLM loop to break the polarization symmetry. Through the QWR position, it is possible to adjust the transmission behavior from a maximum to a minimum at a low input power. With our configuration, it is possible to get self-starting mode-locking operation at a specific position of the QWR. This QWR position corresponds to a value close to the minimal transmission. The pulse repetition frequency was 0.8 MHz. The mode-locked laser ran in a stable operation for hours. We achieved a stable generation of picosecond pulses with milliwatts of an average output power.

A tunable nonlinear optical loop mirror with feedback using linear highly birefringent fiber in the loop

The dynamics of a nonlinear optical loop mirror (NOLM) with feedback using a high birefringence fiber in the loop are investigated. The effect of rotating input polarization angle on the output power and polarization angle is numerically examined, illustrating the sensitivity of the NOLM with feedback to the input’s polarization state, as well as the polarization chaos present with sufficient input powers. The inclusion of a polarization-dependant loss in one or both arms of the coupler is also shown to change the output dynamical behaviour of the system.

Easily tunable nonlinear optical loop mirror based on polarization asymmetry

The operation of an unconventional, power-symmetric nonlinear optical loop mirror (NOLM) is investigated. Its principle is based on the creation of a polarization asymmetry between the counterpropagating beams, through the use of a quarter-wave plate and highly twisted fiber in the loop. Using a very intuitive approach, we propose a simple although comprehensive description of the NOLM operation. By adjusting the angle of the quarter-wave plate, the interferometer can be tuned continuously from non-power-dependent operation to nonlinear switching, in a very convenient way. Experimental results confirm theoretical predictions. The properties of the proposed NOLM design make it very attractive for various applications, like pedestal suppression and amplitude regularization of optical pulse trains.

Metropolitan Access Network

The optical fibre is the highest quality transmission mediumfor broadband services, and therefore the new access cablenetwork has to be realized to the greatest extent and as closeas possible to the subscriber by means of the optical fibre cables,in accordance with the possibilities of the telecommunicationoperator. The development of digital technology in commutations,supported by the increasingly powerful systems of processorcontrol, development of optical communications andtransport technologies in the framework of SDH concepts, arereflected on the access telecommunication networks of the urbanareas. Urban areas are the most profitable regions in telecommunications.In planning of urban optical networks the fact should betaken as a guideline that only a well-organised urban networkwill allow high quality of service provision of the leased cablesto the end points of business customers.Business customers have greater requirements for the networkreliability, flexibility and maintainability, for the sake ofundisturbed telecommunication traffic.The optical medium based technology- FITL (fiber in theloop), provides almost limitless upgrading of the system regardingthe transmission bandwidths, as well as adaptation to allthe future customers' requirements.Considering the increase in the price of installing the coppercables, the prices of electronics and optical components arecontinuously falling. The application of the optical medium isincreasing and becoming more cost-effective, and due to higherreliability of the optical transmission systems the maintenancecosts are reduced.

High-speed data services using the Switched Digital Broadband Access System

The Switched Digital Broadband Access System (SDBAS) from Lucent Technologies (co-developed with BroadBand Technologies, Inc.) offers residential and small business customers access services based on asynchronous transfer mode (ATM) technology. SDBAS is now being deployed by several carriers as a fiber-in-the-loop (FITL) application. With the advent of inexpensive devices from multiple vendors implementing the ATM segmentation and reassembly (SAR) function and interfacing with the peripheral component interconnect (PCI) bus on the personal computer, it is now possible to support cost-effectively general-purpose consumer data services such as Internet access coexisting with other multimedia services. This paper describes the architecture for consumer data deployed as part of a service-independent and intrinsically secure ATM access system such as SDBAS. Features of such a system include support for Internet access; work-at-home (as a service differentiated from Internet access); billing, access control, authentication, and transport layer protocol management using a session and resource management entity; protocols for end point Layer 3 management; and network interworking functions. This paper describes the multi-application service-independent system architecture of an end-to-end high-speed data transport architecture coexisting with other services over the same access system.

Fiber laser mode locked by a Sagnac interferometer with nonlinear polarization rotation

We describe a new fiber laser configuration based on a nonlinear optical loop mirror with a symmetrical coupler, a quarter-wave retarder, and highly twisted, birefringent fiber in the loop. The nonlinear optical loop mirror configuration operates by nonlinear polarization rotation. We have achieved stable generation of subpicosecond pulses with milliwatts of average output power.

Nonlinear optical loop mirror with low birefringence twisted fiber in the loop

The fiber loop mirror constructed from low-birefringence twisted fiber was numerically examined. Nonlinear ellipse rotation effect has been taken into account. Nonlinear switching has been found for the balanced configuration with a 0.5/0.5 coupler. By appropriate choice of the fiber twist, birefringence, and optical fiber length, a variety of different characteristics of the interferometer can be achieved. Some of the configurations manifest polarization independent operation.

Dispersion-managed solitons in a fiber loop with in-line filtering

We investigate both numerically and experimentally soliton propagation in a fiber loop with dispersion management, in-line filters, and frequency shifting. More than 90% of the fiber in the loop is in the normal-dispersion regime, but the net dispersion is anomalous. Stable pulses in the loop have an enhanced power relative to solitons in a fiber with uniform dispersion equal to the loop's path-averaged dispersion. Because the loop's path-averaged dispersion is small, the in-line filtering and the frequency shifting play an important role in pulse shaping. Recirculating loop experiments that demonstrate stable pulse propagation over 28,000 km are consistent with results from computer modeling.

An overview of broad-band access technologies

The provision of broad-band services based on either the Internet or the asynchronous transfer mode (ATM) technique requires a new generation of access networks. In the short term, solutions such as x-digital subscriber line (DSL) and hybrid fiber coaxial (HFC), allowing the reuse of existing infrastructures look very promising. For the longer term, new infrastructures based either on radio or on optical access links seem preferable. Three x-DSL techniques aiming at high-bit-rate transmission over twisted pairs are presented: high-bit-rate DSL, asymmetrical DSL, and very-high-bit-rate DSL. An extension of existing cable television networks known as HFC is also described. Two other prospective approaches, wireless in the loop (WITL) and fiber in the loop (FITL), are then presented. Several techniques are considered for WITL: digital enhanced cordless telecommunication local multipoint distribution service, wireless ATM, and low Earth orbit satellite constellations. The various architectural alternatives for the FITL approach are discussed, a special interest being dedicated to the synchronous digital hierarchy self-healing loop and the ATM over a passive optical network. We mention the main experiments and the standardization activities inherent to the domain.

Broadband Data Services over Residenial Access Networks

ese are truly exciting times for residential and business customers served by local telephone, cable television, and direct satellite providers. Perhaps for the first time, we are witnessing the application of computer networking techniques to telecommunication access network architectures. These local access options include fiber in the loop, fiber to the home, hybrid fibericoax, switched digital video, direct broadcast satellite, wireless microwave/RF links, broadband PCS, and so on. All these state-of-the-art access alternatives are being equipped with an overlaid TCP/IP data services delivery architecture to support high-speed interactive data services to the subscriber’s premises. Transmission rates from a few hundred kilobits per second to tens of megabits per second are envisioned using various digital subscriber line (xDSL over twisted pair) and RF subcarrier multiplexing (cable modems over coax) technologies. The ultimate objective is to equip the end user with high-speed access to the Internet, corporate intranets, and popular on-line services. This is with the grand intention of finally solving the last-mile bandwidth bottleneck and facilitating the deployment of advanced multimedia services such as work at home, distance learning, telemedicine, home shopping, electronic commerce, and high-speed Web access. This special issue is dedicated to provide up-to-date information on research topics, research results, and performance and economic comparisons of various access methods and architectures currently being considered for interactive broadband services (IBS) over residential access networks. The 1996 Telecommunications Deregulation Act allows both local and long-distance telephone service providers and television service providers to offer IBS to the home. Under this intense competition, the winning IBS providers would be those who can either use as much existing infrastructure as possible or cost-effectively and rapidly deploy access, transport, service, and management infrastructures. Research, investigations, standardization efforts, and field trials on all aspects of IBS to the home are progressing at full swing. We received more than 20 articles for publication in this special issue, but could accommodate only seven here. The reviewers selected the ones which fit

Open technical issues in provisioning high-speed interactive data services over residential access networks

This article is intended as an overview of high-speed data services delivery over residential access networks. The author identifies and lists some of the most important open technical issues that deserve further investigation by members of the global technical community prior to a truly large-scale deployment of a wide variety of high-speed interactive multiple-media services over state-of-the-art residential broadband access networks. The target access options include hybrid fiber/coax, fiber in the loop, digital subscriber line, digital satellite, broadband wireless, and so forth.